US President Joe Biden on Saturday castigated Vladimir Putin over the month-old war in Ukraine, bluntly calling the Russian leader "a butcher" who "cannot remain in power".

In an impassioned speech from the Royal Castle in Warsaw, delivered after meeting top Ukrainian ministers in Poland and earlier conferring with NATO and EU allies on the conflict, Biden plainly warned Russia: "Don't even think about moving on one single inch of NATO territory."

Although the White House moved quickly to temper Biden's unprecedented comments on Putin -- insisting the US leader is not seeking "regime change" in Russia and was referring to Putin's influence over neighbours in the region -- the Kremlin made its displeasure clear.

Moscow said it had destroyed Ukraine's largest remaining military fuel depot, at Kalynivka near the capital Kyiv
Moscow said it had destroyed Ukraine's largest remaining military fuel depot, at Kalynivka near the capital Kyiv AFP / FADEL SENNA

Personal attacks, one official said, were "narrowing down the window of opportunity" for bilateral relations.

Biden coupled his harsh words for Putin with a pointed attempt to appeal to ordinary Russians, saying they were "not our enemy" and urging them to blame their president for the heavy sanctions imposed by the West.

He offered reassurance to Ukrainians in the audience and elsewhere, at a time when nearly four million of them have been driven out of their country. "We stand with you," he said.

Biden also cast doubt on Russia's signal that it may scale down its war aims to concentrate on eastern Ukraine -- even as two Russian missile strikes slammed into the west of the country.

Nearly 3.7 million people have fled Ukraine
Nearly 3.7 million people have fled Ukraine AFP / Daniel MIHAILESCU

The president said he was "not sure" Moscow has indeed changed its objectives, which, so far, he said had resulted in "strategic failure".

Two Russian missiles earlier struck a fuel depot in western Ukraine's Lviv, a rare attack on a city just 70 kilometres (45 miles) from the Polish border that has escaped serious fighting.

At least five people were wounded, regional governor Maksym Kozytsky said, as AFP journalists in the city centre saw plumes of thick black smoke.

About 100,000 civilians are said to be trapped in Mariupol with dwindling supplies of food, water and power
About 100,000 civilians are said to be trapped in Mariupol with dwindling supplies of food, water and power Satellite image ©2022 Maxar Technologies via AFP

Putin sent troops into Ukraine on February 24, vowing to destroy the country's military and topple pro-Western President Volodymyr Zelensky.

But his army has made little progress on capturing key cities, and it has hit hospitals, residential buildings and schools in increasingly deadly attacks on civilians.

Smoke rises behind a Ukrainian flag after Russian attacks hit a fuel storage facility in the city of Kalynivka
Smoke rises behind a Ukrainian flag after Russian attacks hit a fuel storage facility in the city of Kalynivka AFP / FADEL SENNA

Biden, who was winding up a whirlwind visit to Poland after holding a series of urgent summits in Brussels with Western allies, earlier met Ukrainian Foreign Minister Dmytro Kuleba and Defence Minister Oleksii Reznikov in Warsaw in an emphatic show of support for Kyiv.

Both ministers had made a rare trip out of Ukraine for the face-to-face talks, in a potential sign of growing confidence in their battle against Russian forces.

In a possible shift on a plan to transfer Soviet-era fighter jets from Poland to Kyiv to boost Ukraine's firepower in the skies -- rejected last month by the Pentagon as too "high risk" -- Kuleba said the United States now did not object.

Evacuees from Mariupol arrive by train in the western Ukrainian city of Lviv
Evacuees from Mariupol arrive by train in the western Ukrainian city of Lviv AFP / Aleksey Filippov

"As far as we can conclude, the ball is now on the Polish side," Kuleba said in written comments to AFP after the meeting.

In a video address, Zelensky reiterated a call for planes while urging allies to supply Ukraine with more weapons.

"We need more ammunition. We need it to protect not only Ukraine but other Eastern European countries that Russia threatened to invade," he said.

"During the meeting... with our American colleagues in Poland, we made it clear again," he said.

US President Joe Biden delivers a speech to members of the US 82nd Airborne Division
US President Joe Biden delivers a speech to members of the US 82nd Airborne Division AFP / Brendan SMIALOWSKI

"What is NATO is doing? Is it being run by Russia? What are they waiting for? It's been 31 days. We are only asking for one percent of what NATO has, nothing more."

US Secretary of State Antony Blinken, meanwhile, announced an additional $100 million in aid to help Ukraine police and border guards purchase armoured vehicles, equipment and medical supplies, a statement said.

Musicians play for people living in a metro station used as a bomb shelter in Kharkiv
Musicians play for people living in a metro station used as a bomb shelter in Kharkiv AFP / Aris Messinis

On the frontlines, Russia's far-bigger military continued to combat determined Ukrainian defenders who are using Western-supplied weapons -- from near the capital Kyiv to Kharkiv, the eastern Donbas region and the devastated southern port city of Mariupol.

In an update early Sunday, the Ukrainian General Staff said "the allied forces repulsed seven attacks" and destroyed eight tanks in the Donetsk and Luhansk areas of the Donbas. It earlier accused Russia of "hiding the real number of personnel and hardware losses".

Russia's defence ministry reported a battle for control of two villages near the separatist stronghold of Donetsk and also claimed a missile strike had destroyed an arms and ammunition depot in the Zhytomyr region, west of Kyiv, on March 25.

A humanitarian convoy leaving the devastated southern port of Mariupol -- including ambulances carrying wounded children -- arrived in Zaporizhzhia after being held up at Russian checkpoints for two days, a Ukrainian official said.

"The ambulances carrying wounded children are also queueing. The people have been deprived of water and food for two days," she wrote on Telegram, blasting Russian troops for "creating obstacles".

Authorities have said they fear some 300 civilians in Mariupol may have died in a Russian airstrike on a theatre being used as a bomb shelter last week, with about 170,000 people still trapped in the besieged city.

It is very difficult to independently verify what is happening on the ground.

In Kharkiv, where local authorities reported 44 artillery strikes and 140 rocket assaults in a single day, residents were resigned to the incessant bombardments.

Anna Kolinichenko, who lives in a three-room flat with her sister and brother-in-law, said they don't even bother to head down to the cellar when the sirens go off.

"If a bomb drops, we're going to die anyway," she said. "We are getting a little used to explosions."

Russian forces have taken control of Slavutych, the town where workers at the decommissioned Chernobyl nuclear plant live, briefly detaining the mayor, regional Ukrainian authorities said.

Residents of the town protested, prompting the invading forces to fire shots in the air and lob stun grenades into the crowd.

The International Atomic Energy Agency said it was "closely monitoring the situation" and was concerned about the ability of employees at Chernobyl to rotate and return to their homes to rest.

"There has been no staff rotation at the NPP for nearly a week now," the IAEA said.

Kyiv's mayor cancelled a planned 35-hour curfew, as Britain's defence ministry said Ukrainian counterattacks were under way near the capital.

"Enemy sabotage groups in Kyiv region are still attempting to penetrate the capital," the Ukrainian General Staff said.

Air-raid sirens sounded early Sunday in Kyiv and several other cities, with residents warned to take shelter.

Ukraine's defence ministry said its forces had recaptured Trostianets, a town near the Russian border that was one of the first to fall under Moscow's control.

Images published by the ministry showed Ukrainian soldiers and civilians among heavily damaged buildings and what appeared to be abandoned Russian military equipment.

In the face of unexpectedly fierce Ukrainian resistance, Russia's army has exhibited poor discipline and morale, suffering from faulty equipment and employing tactics sometimes involving brutality toward civilians, Western analysts say.

British Foreign Secretary Liz Truss on Saturday said UK sanctions against Russia could be lifted if Moscow committed to a full ceasefire and withdrew its troops.

Her comments echoed remarks by US Secretary of State Antony Blinken that the wide-ranging penalties against Russia are "not designed to be permanent" and could "go away" if Moscow changes its behaviour.