Ukrainian President Volodymyr Zelensky called on Saturday for comprehensive peace talks with Moscow to stop its invasion of his homeland, warning it would otherwise take Russia "several generations" to recover from losses suffered in the war.

Since launching the assault on February 24, Russian troops facing tough resistance have largely stalled in their advance, even as they encroach on the capital Kyiv. Looking to regain momentum, they have laid siege to cities and blasted urban areas to rubble.

Air raid sirens sounded early on Saturday in the Kyiv, Chernihiv and Zhytomyr regions, but there were no immediate reports of fresh attacks.

Unprecedented Western sanctions aimed at isolating Russia's economy have done little to halt what President Vladimir Putin calls a "special operation" to reduce its neighbour's military capabilities and "denazify" its government.

After Russia said it was "tightening the noose" around the key port of Mariupol, Ukraine's defence ministry acknowledged on Friday it had "temporarily" lost access to the Azov Sea, which connects to the Black Sea and would be a major loss for Ukraine.

But Western officials say Russia has taken heavy losses with signs of flagging morale among its troops, a trend Zelensky alluded to in a video message in which he urged renewed negotiations to end the war.

"I want everyone to hear me now, especially in Moscow. The time has come for a meeting, it is time to talk," he said early on Saturday. "The time has come to restore territorial integrity and justice for Ukraine. Otherwise, Russia's losses will be such that it will take you several generations to recover."

Russia acknowledged on March 2 that nearly 500 of its soldiers had been killed but has given no update since. Ukraine says the number now amounts to many thousands. Reuters could not independently verify the death count.

Kyiv and Moscow reported some progress in talks this week towards a political formula that would guarantee Ukraine security protection outside of the NATO alliance.

But Ukraine said the need for an immediate ceasefire and withdrawal of Russian troops remained, and each side accused the other of dragging out the talks.

Putin has vowed to continue the invasion until it succeeds, promising tens of thousands of people waving Russian flags at a soccer stadium in Moscow on Friday that Russia "will absolutely accomplish all of our plans."

The United States has repeatedly warned that Russia may turn for assistance to China, the biggest power that has not condemned the assault.

In a video call on Friday, President Joe Biden told China's President Xi Jinping there would be "consequences" if Beijing provides "material support" to Russia in Ukraine, the White House said. It said sanctioning Beijing was an option.

China and Russia deny Beijing is considering giving Moscow military aid. China says it wants to see an end to the conflict.

FLEEING FOR COVER

U.S. oilfield services companies Halliburton Co and Schlumberger said on Friday they have suspended or halted Russia operations in response to U.S. sanctions over Moscow's invasion.

Russia, among the world's largest oil and gas producers, exports 7 million to 8 million barrels per day of crude and oil products. BP PLC , Shell , Equinor ASA and Exxon Mobil have suspended business or announced plans to exit their Russia operations.

In the fourth week of their land, sea and air assault, Russian forces have not captured any big cities and Ukraine's elected government remains defiant in the capital.

Fleeing bombardment that Ukraine says has struck housing blocks, schools, hospitals and cultural facilities, hundreds of thousands of Ukrainians had left for of Lviv in western Ukraine, far from the frontline in the country's east.

Three missiles landed at an airport near Lviv on Friday, raising fears the city could come under a deeper assault.

Civilians sheltering in eastern Ukrainian cities have had to endure cuts to electricity, heating and water, with food and other vital resources increasingly scarce. More than 3 million refugees have fled over Ukraine's western border.

Some of the heaviest fighting has been in Mariupol, where some 400,000 people have been trapped for over two weeks. Officials there say fighting has reached the city centre and that near-constant shelling was preventing humanitarian aid from getting in.

Rescue workers were still searching for survivors of a Mariupol theatre flattened by Russian air strikes on Wednesday. Russia denies hitting the theatre.

In his video address, Zelensky said there was no information about how many people had died in the theatre, where hundreds had been sheltering. More than 130 people had been rescued, he said.

As people continue trying to flee through "humanitarian corridors" arranged by Ukrainian and Russian officials, the World Food Programme has warned that food supply chains in the country are collapsing.

Key infrastructure such as bridges and trains have been destroyed by bombs and many grocery stores and warehouses are empty, said Jacob Kern, WFP emergency coordinator for the Ukraine crisis.

"Inside Ukraine, our job is in effect to replace the broken commercial food supply chains," he said, calling this a "mammoth task".