U.S. President Joe Biden sought to prevent Beijing giving new life to Russia's invasion of Ukraine in a video call on Friday with Chinese President Xi Jinping, as Moscow pressed on with bombardments that have taken the place of military advances.

With Russia trying to regain the initiative in a stalled campaign, three missiles landed at an airport near Lviv, a city where hundreds of thousands thought they had found refuge far from Ukraine's battlefields.

The Russian defence ministry said it was "tightening the noose" around the besieged southern port of Mariupol, where officials said more than 1,000 people may still be trapped in makeshift bomb shelters beneath a destroyed theatre.

Ukraine said it had so far rescued 130 people from the theatre's basement after the building was flattened by Russian strikes two days ago. Russia denies hitting the theatre and says it does not target civilians.

China is the one big power that has yet to condemn Russia's assault and Washington says it fears Beijing may be considering giving financial and military support, something that both Russia and China deny.

Xi said the Ukraine crisis was something China did not want to see, and that Beijing and Washington should guide bilateral relations along the right track, Chinese state media said after he and Biden began their call.

The White house said later the call had lasted just under two hours.

'MINIMAL PROGRESS'

At the start of the fourth week of President Vladimir Putin's invasion to subdue what he calls an artificial state undeserving of nationhood, Ukraine's elected government is still standing and Russian forces have not captured a single big city.

Putin used a rally before the packed Luzhniki soccer stadium in Moscow to justify the invasion, promising tens of thousands of people waving Russian flags that it would succeed.

"We know what we need to do, how to do it and at what cost. And we will absolutely accomplish all of our plans," Putin said, adding that, when needed, Russian soldiers "shield each other from bullets with their bodies like brothers".

Russian troops have taken heavy losses while blasting residential areas to rubble, sending more than 3 million refugees fleeing in what Moscow calls a "special operation" to disarm its neighbour.

"Russian forces have made minimal progress this week," the UK defence ministry said. "Ukrainian forces around Kyiv and Mykolaiv continue to frustrate Russian attempts to encircle the cities."

Jakob Kern, emergency coordinator for the crisis at the U.N. World Food Programme, said Ukraine's "food supply chain is falling apart. Movements of goods have slowed down due to insecurity and the reluctance of drivers".

WFP, which feeds people in global crisis zones, also buys nearly half of its wheat from Ukraine. Kern said the war has already driven global food prices to all time highs, and could cause "collateral hunger" in poor countries worldwide.

The White house said later the call had lasted just under two hours.

INTENSE SHELLING

Russia has been intensively shelling eastern Ukrainian cities, especially Chernihiv, Sumy, Kharkiv and Mariupol.

Kyiv has so far been spared a major assault, with long columns of troops bearing down from the northwest and east halted at the gates in heavy fighting. But residents in the capital have endured nightly deadly missile attacks.

Debris from a missile blew a large crater in the ground in the middle of a residential block where a school was also located in northern Kyiv on Friday, shattering hundreds of windows and leaving debris scattered around the complex.

At least one person was killed, emergency services said. Kyiv mayor said 19 people were injured including four children.

"This is a war crime by Putin," said Lyudmila Nikolaenko, visiting her son, who lived in one of the apartments hit. "They say they aren't hitting regular people, they say we are firing at ourselves."

BIDEN PUTTING PRESSURE ON BEIJING

Peace talks have stepped up this week, with Kyiv, which is demanding a Russian ceasefire and withdrawal, saying Moscow has become more serious as a quick victory slips out of its reach.

Both sides have described progress towards a political formula that would keep Ukraine out of the NATO alliance but protected with some other form of guarantee. Both, however, accused each other on Friday of dragging the talks.

With financial sanctions and diplomatic ostracism cutting Russia off from advanced economies, China is Russia's last big economic lifeline.

Putin and Xi signed a "no limits" friendship pact three weeks before the invasion, in an ostentatious event held on the morning of last month's Winter Olympics opening ceremony. The document repeated some of Russia's grievances over Ukraine.

China has so far been treading a careful line, abstaining in votes over U.N. resolutions condemning Russia while declining to refer to the assault as an invasion criticising the West.

But Washington, which this week announced $800 million in new military aid to Kyiv, now says Moscow wants more from Beijing than just diplomatic cover.

Biden, who described Putin as a "murderous dictator", will make clear to Xi that China "will bear responsibility for any actions it takes to support Russia's aggression", Secretary of State Antony Blinken told reporters.

"Beijing will do everything in its power to avoid having to openly take sides, but its formerly relatively cost-free relationship with Russia has become complicated and is now exposing China to growing geopolitical, economic and reputational risks," said Helena Legarda, Lead Analyst at the Mercator Institute for China Studies.

Hours before the phone call, China sailed an aircraft carrier through the sensitive Taiwan Strait - shadowed by a U.S. destroyer - a person with direct knowledge of the matter said.