KYIV, Ukraine—Any cease-fire that allows Russia to keep Ukrainian territories seized since the invasion in February would only encourage an even wider conflict, giving Moscow a badly-needed opportunity to replenish and rearm for the next round, Ukrainian President Volodymyr Zelensky warned.

“Freezing the conflict with the Russian Federation means a pause that gives the Russian Federation a break for rest,” Mr. Zelensky said in an interview with The Wall Street Journal in the heavily fortified presidential compound in Kyiv on Friday. “They will not use this pause to change their geopolitics or to renounce their claims on the former Soviet republics.”

Mr. Zelensky outlined his views at a critical point in the war, as economic pain from the conflict ripples around the world and Russia’s military advances slow down, with both sides exhausted after five months of bloody fighting. Rising energy and food prices, coupled with the prospect of a Russian natural-gas shutdown this winter, are threatening to push Europe into recession, testing Western resolve just as Moscow seeks to divide the Western alliance.

Russian officials have repeatedly expressed hopes that these economic woes would erode critical American and European support for Ukraine, putting pressure on Kyiv to settle the conflict on terms favorable to Moscow, such as allowing Russia to keep recently conquered swaths of southern and eastern Ukraine.

Clad in his trademark military fatigues, Mr. Zelensky acknowledged the cost exacted by Russia’s invasion on Western businesses and consumers, and he praised the country’s allies for refusing to succumb to Russian blackmail and continuing to enable Ukrainian resistance against a much more powerful foe.

“I am thankful to the people there. It is hard for them, they now have high prices, they suffer discomfort because of this war, because of the crises manufactured by the Russians,” the Ukrainian president said. “But it is a matter of values.” Diplomatic concessions to Moscow today might stabilize the markets somewhat, but would only provide a temporary respite and boomerang in the future, he added.

While fierce Ukrainian resistance repelled Russian forces from Kyiv and other parts of northern Ukraine in March and April, Moscow still controls most of the southern regions of Kherson and Zaporizhzhia. Russia has begun issuing passports to these regions’ residents and switching trade there to rubles, potentially preparing for annexation in coming months. Russian President Vladimir Putin this week accused Mr. Zelensky of not wanting a diplomatic solution that would halt the fighting.

Mr. Zelensky said Ukrainians believe all territories must be liberated first and only then can negotiations begin: ‘The faster we do it, the fewer will die.’

“It is a cachalot that has swallowed two regions and now says: Freeze the conflict,” Mr. Zelensky retorted, comparing Russia to the largest toothed whale. “Then it will rest and in two or three years, it will seize two more regions and say again: Freeze the conflict. And it will keep going further and further. One hundred percent.”

The so-called Minsk agreements that Kyiv signed with Russia under pressure from Germany and France in 2014 and 2015 allowed Moscow to consolidate control over parts of Donbas and to weaken Ukraine, laying the ground for this year’s invasion.

Russian forces have focused in the past two months on capturing Ukrainian-controlled parts of the eastern Luhansk and Donetsk regions, collectively known as Donbas, where Moscow created proxy statelets after first intervening in 2014. While Moscow seized the remainder of the Luhansk region after weeks of heavy fighting earlier this month, Ukraine still controls more than one-third of Donetsk, and Russian troops have made no significant advances there in the past two weeks despite relentless fighting.

Mr. Putin has badly underestimated Ukraine, Mr. Zelensky said: “He opened his mouth, like a python, and thought that we’re just another bunny. But we’re not a bunny and it turned out that he can’t swallow us—and is actually at risk of getting torn apart himself.”

Recent shipments of U.S. and allied weapons, particularly the Himars multiple-launch rocket systems and 155 mm howitzers, have helped to blunt Russia’s offensive in Donbas and stabilize the situation there, Mr. Zelensky said. The Russians used to fire 12,000 artillery shells daily against 1,000 to 2,000 by Ukraine, he said. Now, he added, Ukraine can fire some 6,000 shells a day while Russia is beginning to feel a shortage of ammunition and troops.

This change in the balance of firepower has stemmed Ukrainian casualties, Mr. Zelensky said. At the peak of fighting in May and June, he said, Ukraine was losing between 100 and 200 troops a day; now, it is down to some 30 fatalities a day and around 250 wounded.

“I can tell you exactly, because I live with this every day,” the Ukrainian president said, taking out his smartphone to consult figures just supplied by the military.

While Mr. Zelensky declined to disclose total Ukrainian military losses since the war began, citing military policy, he said they are a number of times lower than Russia’s. Ukraine’s military claims that 39,000 Russian troops have died in Ukraine, while Western estimates are about half that number. Russia last released its casualty figures in March.

Stabilizing the front line in Donbas would allow Ukraine to advance in other directions, Mr. Zelensky said. While military analysts believe Ukraine will give priority to retaking Kherson this summer, Mr. Zelensky declined to discuss where and when Ukraine plans to launch its counteroffensives.

Mr. Putin said earlier this week that Ukrainian and Russian delegations had reached a draft peace agreement at talks in Istanbul in late March and accused Kyiv of reneging on the deal. Mr. Zelensky on Friday dismissed Mr. Putin’s assertion as “total delirium.” Kyiv, the Ukrainian president said, desperately tried to find a diplomatic solution with Moscow before the Feb. 24 invasion, but for three years Mr. Putin wouldn’t even take a phone call from him.

“He came here without talking, killed people, displaced 12 million, and now says Ukraine doesn’t want to negotiate,” Mr. Zelensky exclaimed. “They just murder people, destroy cities, enter them, and then say: ‘Let’s negotiate.’ With whom can they talk? With rocks? They are covered in blood, and this blood is impossible to wash off. We will not let them wash it off.”

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Now, after all the tragedies of the past five months, Ukraine’s citizens are in no mood for talks with Russia, Mr. Zelensky added.

“The society believes that all the territories must be liberated first, and then we can negotiate about what to do and how we could live in the centuries ahead,” he said. “Our people are convinced we can do it. And the faster we do it, the fewer will die.”

“We would prefer to de-occupy in a way that’s not military and to save lives,” he added. “But we are dealing with who we are dealing with. Until they get smashed in the face, they won’t understand anything.”

The Western supplies of Himars, while making a material difference, are much lower than what Ukraine needs to turn the tide, Mr. Zelensky said. An even more urgent need, however, is air-defense systems that could prevent Russia from raining long-range missiles on otherwise peaceful cities hundreds of miles from the front lines, he said. The U.S. and Germany have pledged, but not yet delivered, such large-area systems.

These air-defenses would be much cheaper for the U.S. and Europe to provide than keeping afloat the Ukrainian economy, with its monthly budget shortfall of $5 billion, Mr. Zelensky said. More than six million Ukrainians have fled the country since February, and most of them remain outside the country, mostly in Europe.

“Why doesn’t the economy work? Because people are abroad, women and children,” Mr. Zelensky said. “A woman who comes back with her child while her husband is fighting needs to go to work. But everyone is afraid to send kids to school because of the missiles.”

A 44-year-old former comedian who was elected on an anticorruption campaign with 73% of the vote in 2019, Mr. Zelensky has made a dramatic adjustment to life as a wartime leader since the invasion. Refusing to abandon Kyiv as Russian forces closed in in February, he has remained in the hilltop government area in the city’s Pechersk district that Russian infiltrators tried to attack in the initial days of the war.

Mr. Zelensky, who has left Kyiv only to visit front-line cities since February, stays day and night in the presidential compound, its windows covered by sandbags with armored vehicles parked outside. He said his average day begins with waking up and checking the latest casualty count and the news of Russian missile attacks.

“I used to come to work from my home like any other person, and after work I’d go home,” Mr. Zelensky said wistfully. “I’d wake up, eat, exercise, and come to work…Now I live at work, I wake up at work.”

In his morning meetings, military officials report on the latest situation on the front, ministers tell him about the state of fuel shortages and infrastructure damage from Russian missile attacks, and defense and diplomatic officials brief him on Western weapons supplies.

“It’s about how do we repair railways, bridges, tunnels, how do we outwit the Russians,” he said. Then, Mr. Zelensky said he moves on to diplomatic work, communicating with foreign leaders. Some of the closest allies, he said, are in constant contact with him on WhatsApp.

Late in the evening, when this work is largely done, the Ukrainian president usually sits down to record his daily video address to the nation and the world. Spreading Ukraine’s narrative—and countering Russian propaganda—has been a crucial tool in the war, said Mr. Zelensky, who once ran a TV production company. “This has been a powerful weapon that managed to influence the supply of other kinds of weapons,” he chuckled.

Mr. Zelensky’s frequent speeches, to audiences ranging from parliaments to the Glastonbury music festival to university commencements, aim to put pressure on governments worldwide by appealing directly to their voters—and preventing Ukraine fatigue by keeping the country’s cause in the headlines.

Politicians in democratic nations can sometimes act against public opinion, he said. “But when it’s a question of the fight for freedom, of tyranny, of limiting people’s basic rights, of war, then you don’t have a right to go against society because it has elected you.”

This limitation also applies to Mr. Zelensky. A Wall Street Journal-NORC poll last month found that 89% of Ukrainians oppose a peace deal that would cede any territory conquered by Russia this year.

Write to Yaroslav Trofimov at yaroslav.trofimov@wsj.com and Matthew Luxmoore at Matthew.Luxmoore@wsj.com