About 200,000 people remained trapped in the besieged city of Mariupol on Monday after fighting stopped evacuation efforts over the weekend, with no sign that massive international sanctions were deterring Moscow from its invasion of Ukraine.

Oil prices soared to their highest levels since 2008 in Asian trade after the Biden administration said it was exploring banning imports of Russian oil. Russia provides 7% of global supply.

Japan, which counts Russia as its fifth-biggest supplier of crude oil, is also in discussion with the United States and European countries about possibly banning Russian oil imports, Kyodo News reported on Monday.

Russia's invasion has been condemned around the world, sent more than 1.5 million Ukrainians fleeing abroad, and triggered sweeping Western-led sanctions aimed at crippling the Russian economy.

Most people trapped in the port city of Mariupol are sleeping underground to escape more than six days of shelling by Russian forces that has cut off food, water, power and heating supplies, according to the Ukrainian authorities.

About half of the 400,000 people in the city were due to be evacuated on Sunday but that effort was aborted for a second day when a ceasefire plan collapsed.

Moscow has repeatedly denied attacking civilian areas and says it has no plans to occupy Ukraine.

In a phone call with Turkish President Tayyip Erdogan, Russian President Vladimir Putin said he was ready for dialogue to end the fighting but only if Ukrainian forces stopped resisting and accepted his demands, according to the Kremlin.

The civilian death toll from hostilities across Ukraine since Moscow launched its invasion on Feb. 24 stood at 364, including more than 20 children, the United Nations said on Sunday, adding that hundreds more were injured.

The general staff of Ukraine's armed forces said Russian forces were "beginning to accumulate resources for the storming of Kyiv", after days of slow progress in their main advance on the capital south from Belarus.

'NO TO WAR'

US. Secretary of State Antony Blinken said the United States had seen credible reports of deliberate attacks on civilians in Ukraine, and Washington was documenting them to support a potential war crimes investigation.

Ukrainian President Volodymyr Zelenskiy warned Russians who committed atrocities against civilians they would ultimately face punishment.

"For you there will be no peaceful place on this earth, except for the grave," he said in a televised evening address.

As anti-war protests took place around the world, Ukraine renewed its appeal to the West to toughen sanctions and also requested more weapons, including Russian-made planes.

Blinken said Washington was considering how it could backfill aircraft for Poland if Warsaw decided to supply its warplanes to Ukraine.

He also said the United States and its European allies were exploring banning imports of Russian oil.

Europe relies on Russia for crude oil and natural gas but has become more open to the idea of banning Russian products in the past 24 hours, a source familiar with the discussions told Reuters on Sunday.

Putin says he wants a "demilitarised", "denazified" and neutral Ukraine, and on Saturday likened Western sanctions "to a declaration of war".

New Zealand became the latest country to announce sanctions on Monday, including a plan to stop super yachts, ships and aircraft from entering the country's waters or airspace.

South Korea toughened its financial sanctions against Russia by banning transactions with Russia's central bank.

Australian Prime Minister Scott Morrison urged China to act on its declarations of promoting world peace and join the effort to stop Russia's invasion of Ukraine, warning that the world was in danger of being reshaped by an "arc of autocracy".

"No country will have a bigger impact on concluding this terrible war in Ukraine than China," Morrison said in response to a question after a speech at the Lowy Institute think tank.

Western sanctions have pushed many companies to exit investments in Russia, while some Russian banks have been shut out of a global financial payment systems, driving down the rouble and forcing Moscow to jack up interest rates.