In the fighting that has devolved into a back-and-forth stalemate, Ukrainian forces retook Irpin, a key suburb northwest of the capital, Kyiv, President Volodymyr Zelenskyy said late Monday, but he warned that Russian troops were regrouping to take the area back.

"We still have to fight, we have to endure," Zelenskyy said in his nighttime video address to the nation. "This is a ruthless war against our nation, against our people, against our children."

The authorities of Irpin said Monday that the city had been "liberated" from Russian troops. As well as Irpin, Ukrainian forces also seized back control of Trostyanets, south of Sumy in the northeast, after weeks of Russian occupation that has left a landscape devastated by war.

"The occupiers have been pushed away from Irpin and Kyiv. But it’s still too early to talk about safety in this part of our region. Fighting continues," Zelenskyy said in his nightly video address. "Russian troops hold the north of Kyiv region under control. They have resources and manpower. They are trying to rebuild destroyed units."

Arriving in Trostyanets Monday shortly afterward, The Associated Press (AP) saw the bodies of two Russian soldiers lay abandoned in the woods and Russian tanks lay burned and twisted. A red "Z" marked a Russian truck, its windshield fractured, near stacked boxes of ammunition. Ukrainian forces piled atop a tank flashed victory signs. Dazed residents lined up amid charred buildings seeking aid.

It was unclear where the Russian troops went, under what circumstances they fled and whether the town will remain free of them. In his overnight address, Zelenskyy emphasized the situation remains tense in Ukraine’s northeast around Kharkiv, the nearest large city, and other areas, as he pressed Western countries to do more to support Ukraine, including providing them with more weapons and levying harsher sanctions on Russia.

"If someone is afraid of Russia, if he or she is afraid to make the necessary decisions that are important to us, in particular for us to get planes, tanks, necessary artillery, shells, it makes these people responsible for the catastrophe created by Russian troops in our cities, too," he said. "Fear always makes you an accomplice."

But the returned presence of Ukrainian forces in Trostyanets was a relief for a country hoping that Russian forces are pulling back as they encounter fierce resistance.

A senior U.S. defense official said Washington believes the Ukrainians have retaken Trostyanets. The official, who spoke on condition of anonymity to discuss U.S. intelligence assessments, said Russian forces largely remained in defensive positions near the capital, Kyiv, and were making little forward progress elsewhere in the country.

In Trostyanets, after weeks of occupation and intense fighting, some residents appeared to have lost all sense of normal.

"Personally, I have not seen much," said one resident, Vitali Butski. And yet three missiles struck his home. Many buildings beyond the railway station are damaged, he said.

Bundled up against the freezing wind, he and others ventured out to see what had been left behind. Unexploded ordnance littered the square in front of the train station. Trenches and berms lined the square in a sign that Russian forces tried to defend their position. In a bunker under the station, with thick walls and doors, rooms were full of army uniforms and boots left behind.

On the walls were patriotic messages, including drawings signed by children in Russian reading, "Thanks for the peace, soldier." Another room had been used as a clinic, with unused drips ready and desks turned into beds, although there was no sign of blood.

Packets of Russian food rations were seen amid the debris. But residents indicated that the soldiers were still hungry.

"In the evenings they came to us, to our houses and our basements, and stole our pickles, potatoes, lard and cucumbers," said one resident who didn’t give her name.

She called the Russians "orcs," or goblin-like creatures. Militias from the Donetsk and Luhansk regions were there as well, she said. The entire town had been occupied. Now, for residents, there is some space to breathe. Standing in line for aid, they waved to passing Ukrainian tanks.

"As you can see, there were battles here over the past month. Projectiles were flying over, and people were saying they were frightened," said Evgeni Kosin with the emergency services. "They were left without food and water. There was a horrible humanitarian situation. Now that there are no flyovers or shelling in the last three days, perhaps it is getting better."

Russian President Vladimir Putin’s ground forces have become bogged down because of the stronger-than-expected Ukrainian resistance, combined with what Western officials say are Russian tactical missteps, poor morale, shortages of food, fuel and cold weather gear, and other problems.

In response, Russia appeared to be concentrating more on Donbass, the predominantly Russian-speaking region where Moscow-backed rebels have been waging a separatist war for eight years, the official said.

However, Ukrainian officials still believe that Russia wants to take the capital Kyiv, dismissing suggestions the Kremlin is focused on Donbass region. Capturing "Kyiv is essentially a captured Ukraine, and this is their goal," said Deputy Defense Minister Ganna Malyar, insisting Russia was still "trying to break through the corridor around Kyiv and block transport routes."

On Monday Russian attacks near Kyiv cut power to more than 80,000 homes, officials said, underscoring the continued peril facing the capital.

While that raised a possible face-saving exit strategy for Putin, it has also raised Ukrainian fears the Kremlin aims to split the country, forcing it to surrender a swath of its territory.

Russia has destroyed more than 60 religious buildings across the country in just over a month of the war, with most of the damage concentrated near Kyiv and in the east, Ukraine’s military said in a post on Tuesday. It said the Orthodox Church – the country’s majority religion – was the most affected but that mosques, synagogues, Protestant churches and religious schools were also destroyed.

Humanitarian corridors open

In the besieged southern port of Mariupol, the mayor said half the pre-war population of more than 400,000 has fled, often under fire, during weeks of shooting and shelling. Russian forces have encircled the city and have embarked on a steady and indiscriminate bombardment, trapping an estimated 160,000 people with little food, water or medicine. At least 5,000 people have already died, according to one senior Ukrainian official who estimated the real toll may be closer to 10,000 when all the bodies are collected.

The Ukrainian government says it is operating three humanitarian corridors Tuesday to move civilians out of the besieged port of Mariupol and two Russian-occupied cities in the south.

Deputy Prime Minister Iryna Vereshchuk says that besides Mariupol, evacuations will run from Enerhodar and Melitopol. Those cities have both been under Russian control for weeks and have seen protests and alleged kidnappings of pro-Ukraine local politicians.

"Three humanitarian corridors were agreed for today, Vereshchuk said in a video statement posted on Telegram, a day after announcing their closure, citing intelligence reports. The routes all converge in the Ukraine-controlled southern city of Zaporizhzhia.

Russia operates its own evacuation routes, which Ukraine has alleged are covered for forcibly deporting Ukrainian civilians to Russia. Ukraine and Russia have accused each other of obstructing evacuations.

Alina Beskrovna, who escaped the city in a convoy of cars and made it to Poland, said desperate people are melting snow for water and cooking on open fires despite the risk of bombardment, "because if you don’t, you will have nothing to eat."

"A lot of people are just, I think, starving to death in their apartments right now with no help," she said. "It’s a mass murder that’s happening at the hands of the Russians."

A Ukrainian official says an explosion struck a nine-story administration building in the strategic port city of Mykolaiv on Tuesday morning as talks between Ukrainian and Russian delegations in Istanbul began.

The Telegram channel of the regional governor, Vitaliy Kim, showed a gaping hole in the center of the building. Kim said most people escaped the building and rescuers were searching for a handful of missing people.

The United Nations chief has launched an initiative to immediately explore possible arrangements for "a humanitarian cease-fire in Ukraine" in order to allow the delivery of desperately needed aid and pave the way for serious political negotiations to end the monthlong war.

U.N. Secretary-General Antonio Guterres said Monday he asked Undersecretary-General Martin Griffiths, the head of the U.N.'s worldwide humanitarian operations, to explore the possibility of a cease-fire with Russia and Ukraine. He said Griffiths has already made some contacts.

The 193-member U.N. General Assembly, by an overwhelming majority of about 140 nations, has called for an immediate cessation of hostilities in Ukraine twice – on March 2 and on March 24 – and Guterres told reporters he thinks "this is the moment" for the U.N. "to assume the initiative."

Since Russia's invasion of Ukraine on Feb. 24, the secretary-general said there has been a "senseless loss of thousands of lives," displacement of 10 million people, systematic destruction of homes, schools, hospitals and other essential infrastructure, "and skyrocketing food and energy prices worldwide."