US President Joe Biden came up-close to the war in Ukraine Friday as Russia appeared to row back on its vaulting ambitions, after a month of dogged resistance by Ukrainian forces backed by ever-rising Western support.

Biden visited Poland after forging a new set of measures with the EU designed to lessen Europe's dependence on energy imports from Russia's sanctions-hit economy.

His visit near the border came as a clearer scale of the ruin emerged from Ukraine's besieged port city of Mariupol, which a month into the invasion resembles World War II scenes of Russian cities razed by the Nazis.

Authorities fear some 300 civilians may have died in a Russian air strike on a theatre-turned-bomb shelter in Mariupol last week, in what would be the invasion's single bloodiest attack.

"I have escaped, but I have lost all my family. I have lost my house. I am desperate," Oksana Vynokurova, 33, told AFP after finally escaping Mariupol by train to the western city of Lviv.

"My mum is dead. I left my mother in the yard like a dog, because everybody's shooting," she said.

Also disembarking from the train, Svetlana Kuznetsova said: "There is no water, light and electricity. We were living in cellars. We were cooking food on fires.

"I have never seen such horror. There is no Mariupol," the middle-aged woman added. "Mariupol is like Grozny (in Chechnya). Everything is destroyed."

New Russian focus

Smaller-scale strikes continued without pause as Russia, suffering heavy losses and meagre progress against key targets, pursues a relentless campaign of bombardment against Ukraine's cities.

Giving only its second death toll of the war, the Russian army said it had suffered 1,351 fatalities in the invasion. Ukraine and Western intelligence say it is many thousands more.

In a potentially significant shift, the Russian army said the first phase of its campaign was over and its troops would now focus on the "liberation" of the Russian-speaking Donbas region in Ukraine's east.

Sergei Rudskoi, chief of the Main Operational Directorate of the General Staff of Russia's armed forces, said the shift was possible because "the combat potential of Ukraine's armed forces has been significantly reduced".

Ukraine's Foreign Minister Dmytro Kuleba, however, indicated no let-up in his country's refusal to accede to Russian demands after what he termed "very difficult" talks with Moscow.

"We insist, first of all, on a ceasefire, security guarantees, and territorial integrity of Ukraine," he said.

And while Mariupol and other places are now charred ruins, Western systems including shoulder-fired anti-tank missiles have helped Ukraine's armed forces hold their line -- and increasingly to go on the offensive.

"Ukrainian counter-attacks, and Russian forces falling back on overextended supply lines, has allowed Ukraine to re-occupy towns and defensive positions up to 35 kilometers (22 miles) east of Kyiv," Britain's defense ministry said in a daily update.

Further east, Russian strikes targeting a medical facility in Ukraine's second city Kharkiv killed four civilians and wounded several others, police said.

Several residents told AFP that cluster munitions were used in Kharkiv, spraying death indiscriminately.

Chemical arms warning

After summits of NATO, the European Union and G7 in Brussels, Biden warned that the NATO alliance would "respond" if Russian President Vladimir Putin resorts next to chemical weapons.

En route to Poland, National Security Adviser Jake Sullivan said Russia would pay a "severe price" -- but stressed "the United States has no intention of using chemical weapons, period, under any circumstance".

Kremlin spokesman Dmitry Peskov accused Biden of seeking to "divert attention", and also denied Ukrainian claims that Russia had broken international law by dropping incendiary phosphorus bombs on civilians.

Biden and EU commission chief Ursula von der Leyen announced a joint energy task force in Brussels, before he headed to the eastern Polish town of Rzeszow, a mere 80 kilometers (50 miles) from Ukraine.

Taken together, Western sanctions are "draining Putin's resources to finance this atrocious war", von der Leyen told reporters alongside Biden.

Germany, Moscow's biggest customer in Europe, said it would halve Russian oil imports by June and end all coal deliveries by the autumn.

"The first important milestones have been reached to free us from the grip of Russian imports," Economy Minister Robert Habeck said.

In Poland, Biden met members of the US 82nd Airborne Division, part of NATO's increasingly muscular deployment to its eastern flank.

He was also being briefed on the dire humanitarian situation in Ukraine, which more than 3.7 million people have fled, mostly to Poland.

The UN believes that more than half of Ukraine's children have already been driven from their homes -- "a grim milestone that could have lasting consequences for generations to come", according to Unicef chief Catherine Russell.

Birthday on a bus

In the flashpoint town of Irpin on Kyiv's north-western outskirts, Daria played with her dinosaur mittens as an evacuation bus took her family and others away. It was her fourth birthday on Thursday.

"We were planning some candles and a cake, but we had to leave it there," said Daria's mother Susanna Sopelnikova, 29, holding her tightly on her lap.

"We stayed in the basement for about three weeks, then we decided to leave," Sopelnikova said, to the distant boom of shelling.

Russia said it had destroyed Ukraine's largest remaining military fuel depot, at Kalynivka near Kyiv, using sea-borne cruise missiles.

Fireballs leapt into the air from the storage facility, while a smaller fire blazed from a severed fuel line and a huge plume of black smoke rose over the site, AFP reporters at the scene said.

Cancelling Russia?

For his part, Putin accused the West of discriminating against Russian culture, likening it to the ceremonial burning of books by Nazi supporters in the 1930s.

"Today they are trying to cancel a thousand-year-old country -- I am talking about the progressive discrimination against everything connected with Russia," he said in televised remarks.

After the Kremlin imposed an information blackout on its "special military operation", most Russians are unaware of the true picture of fighting in Ukraine.

But an exhibition of 24 shocking images opened on Friday at a train station in Lithuania used by Russians transiting from the exclave of Kaliningrad.

On some of the pictures, exhibited at the height of the carriage windows, an inscription read: "Today, Putin is killing the peaceful population of Ukraine. Do you approve of this?"

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