President Volodymyr Zelenskyy triumphantly walked the streets of the newly liberated city of Kherson on Monday, hailing Russia’s withdrawal as the “beginning of the end of the war,” but also acknowledging the heavy price Ukrainian troops are paying in their grinding effort to push back the invaders.

The retaking of Kherson was one of Ukraine’s biggest successes in the nearly 9-month-old war, dealing another stinging blow to the Kremlin. It could serve as a springboard for more advances into occupied territory.

President Joe Biden called it a “significant victory” for Ukraine.

“I can do nothing but applaud the courage, determination and capacity of the Ukrainian people, the Ukrainian military,” he said on the sidelines of a Group of 20 summit in Indonesia. “I mean, they’ve really been amazing. And I think it’s hard to tell at this point exactly what it means. … But I’ve been very clear that we’re going to continue to provide the capability for the Ukrainian people to defend themselves.”

Large parts of eastern and southern Ukraine are still under Russian control, and the city of Kherson itself remains within reach of Moscow’s shells and missiles. Heeavy fighting continued elsewhere in the country.

In Kherson, Zelenskyy awarded medals to soldiers and posed with them for selfies while striking a defiant note.

“This is the beginning of the end of the war,” he said. “We are step by step coming to all the temporarily occupied territories.”

But he also grimly noted that the fighting “took the best heroes of our country.”

The end of Russia’s occupation of the city — the only provincial capital its forces have seized since the February invasion — has sparked days of celebration. But with winter approaching, its residents are without heat, water and electricity, and short on food and medicine. Zelenskyy added that the city is laced with booby traps and mines. And Ukrainian authorities say there are signs of atrocities emerging, just as in other liberated areas.

The Institute for the Study of War said Ukraine has won “an important victory” in recapturing the city and other areas west of the Dnieper River, but the Washington-based think tank noted that “it has by no means liberated the minimum territory essential to its future security and economic survival.”

NATO Secretary-General Jens Stoltenberg, while visiting The Hague, warned that “we should not make the mistake of underestimating Russia.”

“The Russian armed forces retain significant capability as well as a large number of troops, and Russia has demonstrated their willingness to bear significant losses,” he said.

In Ankara, Turkey, CIA Director Bill Burns met with his Russian intelligence counterpart, Sergei Naryshkin to underscore the consequences if Moscow were to deploy a nuclear weapon in Ukraine, according to a White House National Security Council official.

The official, who was not authorized to comment publicly and spoke on the condition of anonymity, said Burns and Naryshkin, the head of Russia’s SVR spy agency, did not discuss settlement of the war. Their meeting was the highest-ranking face-to-face engagement between U.S. and Russian officials since before Russian President Vladimir Putin ordered the invasion. Ukrainian officials were briefed ahead of Burns’ travel to Turkey, the official said.

While U.S. officials for months have warned of the prospect that Russia could use weapons of mass destruction in Ukraine amid battlefield setbacks, Biden administration officials have repeatedly said nothing has changed in U.S. intelligence assessments to suggest that Putin has imminent plans to deploy nuclear weapons, according to U.S. officials.

Zelenskyy’s trip to Kherson was another is a series of unexpected visits to front-line areas at crucial moments of the war. This one was laden with both symbolism and the common touch — clearly aimed at boosting morale of soldiers and civilians alike.

In video posted by a presidential aide, a visibly moved Zelenskyy stood with his hand on his heart and sang the national anthem as troops saluted and stood at attention and a soldier steadily raised the yellow-and-blue Ukrainian flag.

People with flags draped around their shoulders cheered, cried and shouted in gratitude as Zelenskyy walked by.

“It’s amazing. We’ve been waiting for him for nine months. Thank you,” said resident Danila Yuhrenko.

Serhii Yukhmchuk, 47, said he and his wife spent the occupation mostly at home to avoid interacting with the Russians. They and others silently protested by refusing to use the Russian ruble as currency in the city, he said.

Kremlin spokesman Dmitry Peskov refused to comment on Zelenskyy’s visit, saying only that “you know that it is the territory of the Russian Federation.” Russia illegally annexed the Kherson region and three other provinces earlier this year.

In his Sunday night address, Zelenskyy said without giving details that “investigators have already documented more than 400 Russian war crimes, and the bodies of both civilians and military personnel have been found.”

“In the Kherson region, the Russian army left behind the same atrocities as in other regions of our country,” he said. “We will find and bring to justice every murderer. Without a doubt.”

Residents said Russian troops plundered the city as they departed last week. They also wrecked key infrastructure before retreating across the wide Dnieper River to its east bank.

One Ukrainian official described the situation in Kherson as “a humanitarian catastrophe.”

Reconnecting the electricity supply is the priority, regional Gov. Yaroslav Yanushevych said.

The arrival of winter is making the situation more difficult, with Stoltenberg saying that Putin is aiming “to leave Ukraine cold and dark this winter.”

Biden said he expected things to slow down somewhat militarily “because of the winter months and the inability to move as easily around the country.”

In the past two months, Ukraine’s military claimed to have retaken dozens of towns and villages north of the city of Kherson, which is a key gateway to the Crimean Peninsula to the south — a region illegally annexed by Russia in 2014.

But the grinding war continued — with shelling, civilian casualties and each side reporting gains.

The Russian Defense Ministry said its forces had completely captured the village of Pavlivka in the eastern Donetsk region. Multiple Ukrainian officials have reported heavy battles in the area in recent weeks but did not confirm the loss of Pavlivka.

In Luhansk, another eastern region illegally annexed by Moscow, Kyiv’s forces have retaken 12 settlements, said regional Gov. Serhiy Haidai.