Smoke rises after an airstrike, as Russia's attack on Ukraine continues, in Lviv, Ukraine, on Saturday. Reuters

US President Joe Biden called Russian leader Vladimir Putin a butcher who "cannot remain in power" after meeting Ukrainian refugees in Poland, as Kremlin forces stepped up attacks across Ukraine, including the western city of Lviv.

Biden's comments, an escalation of US rhetoric towards Moscow over its invasion of Ukraine, were not a call for regime change in Russia, a White House official said later, but meant to prepare the world's democracies for an extended conflict.

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Ukraine's president Zelensky seeks action from Muslims

Russia reframes war goals in Ukraine

As the fight since Russia's Feb. 24 invasion of its neighbour drags on, a visibly irritated Ukrainian President Volodymyr Zelensky again demanded Western nations send military hardware.

US President Joe Biden delivers a speech at the Royal Castle in Warsaw, Poland, on Saturday. AP

Zelensky asked whether the West was intimidated by Russia, demanding they provide a fraction of the military hardware in their stockpiles. "We've already been waiting 31 days."

Biden, in a fiery speech ending a European trip aimed at bolstering Western resolve, framed the war as part of a historic struggle for democratic freedoms.

Four missiles hit Lviv just 60 km (40 miles) from the Polish border, local officials said, in the most significant attack on the city in the month-old war.

People watch smoke rising behind buildings following explosions in Lviv, western Ukraine, on Saturday. AP

Russian forces have taken Slavutych, where workers at the defunct Chernobyl nuclear plant live, an official said.

Russian Defence Minister Sergei Shoigu was shown chairing a meeting, the first time he has publicly been seen speaking for more than two weeks.

Meanwhile, Russian forces took control of a town where staff working at the Chernobyl nuclear site live and briefly detained the mayor, sparking protests, Ukrainian officials said Saturday.

Clothes thrown by the Russian shelling from the ruined house hang on a tree in Kharkiv, Ukraine. AP

"I have been released. Everything is fine, as far as it is possible under occupation," Yuri Fomichev, mayor of Slavutych, told the media by phone, after officials in the Ukraine capital Kyiv announced earlier he had been detained.

Earlier, the military administration of the Kviv region, which covers Slavutych, announced that Russian troops had entered the town and occupied the municipal hospital.

They also said that the mayor had been detained.

Residents took to the streets, carrying a large blue and yellow Ukrainian flag and heading towards the hospital, the administration said. Russian forces fired into the air and threw stun grenades into the crowd, it added.