Moscow and Kyiv traded blame over a failed ceasefire plan that would have let civilians flee Mariupol and Volnovakha, two southern cities besieged by Russian forces.

Another round of talks was tentatively planned for Monday as Ukrainians who could escape spilled into neighbouring Poland, Romania, Slovakia and elsewhere.

In a televised address on Saturday night, Ukraine's President Volodymyr Zelenskiy called on people in areas occupied by Russian troops to go on the offensive and fight.

"We must go outside and drive this evil out of our cities," he said, vowing to rebuild his nation.

British military intelligence said on Sunday that Russian forces were targeting populated areas in Ukraine but that the strength of resistance was slowing their advance.

"The scale and strength of Ukrainian resistance continue to surprise Russia," British military intelligence said in an update.

Russia has repeatedly denied that it is targeting civilian areas.

Russian President Vladimir Putin earlier reiterated that he wanted a neutral Ukraine that had been "demilitarised" and "denazified," and likened Western sanctions "to a declaration of war," adding: "Thank God it has not come to that."

Ukraine and Western countries have decried Putin's reasons as a baseless pretext for the invasion he launched on February 24 and have imposed sweeping sanctions aimed at isolating Moscow and crippling its economy.

Ukraine's Foreign Minister Dmytro Kuleba, after meeting with US Secretary of State Antony Blinken at the Ukraine-Poland border, said he expected new sanctions and weapons for Ukraine in coming days.

The United States has said it would give Ukraine more weapons and has repeatedly warned it could escalate sanctions, with President Joe Biden seeking $US10 billion ($A14 billion) in emergency funding to respond to the crisis.

Washington is working with Poland as Warsaw considers whether to provide fighter jets to Ukraine, a White House spokesperson said.

Zelenskiy had asked for help securing aircraft from European allies in a video call with US lawmakers earlier on Saturday. He also called again for more lethal aid, a ban on Russian oil and a no-fly zone and an end to Visa and MasterCard privileges in Russia.

Visa and Mastercard later said they would suspend credit card operations in Russia.

Biden spoke with Zelenskiy for about 30 minutes on Saturday evening in Washington as Sunday broke in Ukraine, the White House said.

They discussed security, financial support for Ukraine and the continuation of sanctions against Russia, Zelenskiy wrote on Twitter.

Seeking to mediate, Israeli Prime Minister Naftali Bennett met with Putin at the Kremlin on Saturday and later spoke to Zelenskiy, Bennett's spokesperson said.

"We continue dialogue," Zelenskiy tweeted after the call.

Turkish President Tayyip Erdogan is expected to talk with Putin on Sunday. Turkey, a NATO member, shares a maritime border with Ukraine and Russia in the Black Sea.

Russia was warning the EU and NATO again to stop the "pumping of state-of-the-art weapons systems" into Kyiv, foreign ministry spokesperson Maria Zakharova said, according to RIA.

Russian media cited an unidentified source on Sunday as saying that Ukraine was close to building a plutonium-based "dirty bomb" nuclear weapon, although the source cited no evidence.

Russia's Defence Ministry said its forces were carrying out a wide-ranging offensive in Ukraine and had taken several towns and villages, Russian news agency Interfax said.

The Ukrainian armed forces said on Sunday they were conducting a number of defensive operations, including in the eastern part of the Donetsk district, in Slobozhansky, and the city of Chernihiv.

A series of blasts were heard overnight in Kharkiv, the second-largest city, Ukrainian media reported.

Ukraine's military said it had shot down 88 Russian aircraft and helicopters so far and captured some pilots. Reuters could not corroborate the claim.

Concerns over nuclear dangers remained after Russia seized Ukraine's Zaporizhzhia nuclear power plant, with a top US official saying on Friday that Russian troops were 30km from Ukraine's second largest nuclear facility.

The World Health Organisation said 249 civilians had been killed and 553 injured as of Thursday, when the number of refugees was 1.2 million and 160,000 people had been internally displaced. But it added the toll was likely much higher.

The UN refugee agency estimated the number of refugees could swell to 4 million by July.

The International Monetary Fund warned the conflict would have a "severe impact" on the global economy, driving up energy and grain prices.

It said it would weigh Kyiv's request for $US1.4 billion ($A1.9 billion) in emergency financing as early as next week

Australian Associated Press