Ukraine's President Volodymyr Zelensky warned Russia is consolidating and preparing "powerful strikes" in the country's east and south, including besieged Mariupol, where a new attempt will be made Friday to evacuate civilians from the devastated city.

Russia meanwhile threatened to turn off its gas taps to Europe if payments are not made in rubles, as US President Joe Biden ordered a record release of strategic oil reserves to ease soaring US prices.

Over a month into Russia's invasion of its neighbour, Vladimir Putin's troops have devastated cities like Mariupol with shelling, killing at least 5,000 people in the port city alone. 

In peace talks this week, Russia said it would scale back attacks on the capital Kyiv and the city of Chernigiv, but Ukrainian and Western officials have dismissed the pledge, saying Moscow's troops were merely regrouping.

"This is part of their tactics," said Zelensky in a late-night address.

"We know that they are moving away from the areas where we are beating them to focus on others that are very important... where it can be difficult for us," he said.

In particular, he warned, the situation in the country's south and east was "very difficult".

"In Donbas and Mariupol, in the Kharkiv direction, the Russian army is accumulating the potential for attacks, powerful attacks," he said.

Washington echoed that assessment, with a senior US defence official saying Russia's focus on Donbas could herald a "longer, more prolonged conflict".

Military experts believe that Moscow is ditching efforts to advance simultaneously along multiple axes in the north, east and south, after struggling to overcome stronger-than-expected Ukrainian resistance.

Instead it wants to establish a long-sought land link between Crimea, which Moscow occupied in 2014, and the two Russian-backed Donbas statelets of Donetsk and Lugansk.

- 'Civilians desperately wanting to flee' -

Mariupol is the main remaining obstacle to that ambition, and Russian forces have encircled and relentlessly bombarded the city to try to capture it.

Instead, it has been reduced to rubble, with tens of thousands of civilians trapped inside with little food, water or medicine.

Previous attempts to evacuate residents have collapsed, though some have made the dangerous dash to freedom alone, but on Friday Russia says it will allow a humanitarian corridor organised by the International Committee of the Red Cross (ICRC).

The ICRC's Ukraine delegation said on Twitter it was in nearby Zaporizhzhia, where buses from the encircled city are meant to arrive.

"We hope to be able to facilitate safe passage for civilians desperately wanting to flee Mariupol. We are also here with two trucks of assistance, hoping that we can also get assistance in," the organisation's Lucile Marbeau said in a video.

"In these trucks there is food, medicine, relief items, for those civilians who decide to stay," she added.

Ukrainian officials on Thursday sent dozens of buses towards Mariupol, and the local government said on Telegram that civilians would be able to start boarding Friday morning in neighbouring Berdiansk.

Russia has moved about 20 percent of its troops from around Kyiv but its strikes have continued and troops are likely "going to be repositioned, probably into Belarus, to be refitted and resupplied and used elsewhere in Ukraine," said Pentagon spokesman John Kirby

Russian troops have also pulled back from the Chernobyl nuclear plant after weeks of occupation, but have taken a number of captive Ukrainian servicemen with them, according to officials in Kyiv.

Western intelligence has warned that Putin's advisors may be "afraid to tell him the truth" about battlefield losses or the damage that sanctions have wrought on the country's economy.

And Biden suggested Putin may have placed some advisors under house arrest, though he cautioned "there's a lot of speculation."

- 'I hope all this will end soon' -

The Kremlin has rejected the claims, with spokesman Dmitry Peskov saying Western officials "don't understand President Putin, they don't understand the mechanism for taking decisions and they don't understand the style of our work".

With his economy crippled by unprecedented international sanctions, Putin has sought to leverage Russia's status as an energy power, and warned Thursday that EU members will need to set up ruble accounts from Friday to pay for Russian gas.

"If such payments are not made, we will consider this a breach of obligations on the part of our buyers" and existing contracts would be stopped, Putin said.

The EU has joined the United States in imposing sanctions, and European Parliament President Roberta Metsola is expected in Kyiv soon in a show of support.

But the bloc has not imposed an energy embargo, and Germany, which imported 55 percent of its gas supplies from Russia before the war, insisted it will pay in euros or dollars as stipulated in contracts.

Berlin and Paris were also "preparing" for Russian gas to simply stop flowing, France's economy minister said.

Biden meanwhile moved to mitigate rising domestic fuel prices by announcing a release from strategic US reserves of a million barrels daily for six months.

The record release amounts to augmenting global supplies by about one percent.

Peace talks between Ukrainian and Russian officials were expected to resume via video on Friday, with Kyiv negotiator David Arakhamia saying Turkey and Germany had offered to serve as security guarantors in any eventual agreement.

On the ground around Kyiv, Ukrainian forces have continued to push back Russian troops, capturing territory on the outskirts of the capital as Moscow's advance stalls.

Zelensky praised the advances, but said he was stripping two generals of their ranks for unspecified offences.

"Right now I don't have time to deal with all the traitors, but gradually all of them will be punished," he said.

Civilians have been trickling out of devastated areas, including three-year-old Karolina Tkachenko, who was helped over a pipeline east of Kyiv by Ukrainian troops as she and her family escaped.

"The shops are closed, there's no delivery of supplies. The bridge is also blown up, we can't go for the groceries through there," said her mother Karina, holding her daughter in a pink bobble hat in her arms.

"I hope all this will end soon, and I will go back to my work."

Latest developments

Here are the latest developments in the war in Ukraine:

- EU to urge China rethink on Russia -

EU is set to hold a virtual summit with China against a backdrop of increasing alarm over Beijing's growing proximity with Moscow and its reluctance to condemn its invasion of Ukraine.

"The meeting will focus on the role we are urging China to play, to be on the side of the principles of international law without ambiguity and exert all the necessary influence and pressure on Russia," says French European affairs minister Clement Beaune.

- EU parliament leader visiting Kyiv -

European Parliament President Roberta Metsola says she is on her way to Ukraine, making her the first EU leader to visit the war-torn country.

The Maltese MEP, who was elected in January, tweets "On my way to Kyiv" alongside a Ukrainian flag, but gives no further details.

- Russia leaving Chernobyl with hostages -

Russian forces have begun to pull out of the defunct Chernobyl nuclear power site and move towards Belarus, but took an unspecified number of captive Ukrainian servicemen with them, officials in Kyiv say.  

Troops seized control of the Chernobyl site -- where radioactive waste is still stored -- on February 24, the first day of the invasion.

- Putin may be 'isolated': Biden -

US President Joe Biden says that Russian counterpart Vladimir Putin may be "isolated" and could have placed some of his advisors under "house arrest".

In his first public remarks on Western assessments of Kremlin tensions over the war in Ukraine, Biden also says he is "sceptical" about Moscow's claim to be scaling back its onslaught in parts of the country.

- New gas war front -

Putin says "unfriendly" countries, including all EU members, must set up ruble accounts to pay for gas deliveries from April, or "existing contracts would be stopped". 

German Chancellor Olaf Scholz insists payments continue in euros or dollars, while France says Paris and Berlin are preparing for a cut in Russian gas deliveries.

- Buses en route to Mariupol - 

Ukraine's government sends 45 buses to evacuate civilians from the besieged city of Mariupol, where Russia has declared a local ceasefire following a global outcry over the suffering of civilians trapped by a month of relentless shelling.

The first convoy arrives in the Russian-occupied port of Berdyansk, a hub in the operation to evacuate civilians to the Ukrainian-controlled town of Zaporizhzhia.

- 'Longer' conflict possible: US -

Russia's refocusing of its military efforts on the Donbas could herald a "longer, more prolonged conflict" as Ukrainian forces put up fierce resistance there, a senior US defence official says.

"It's been fought over now for eight years," the official says of the heavily contested region.

- Putin ratings up -

Putin's ratings have received a boost since the start of military actions in Ukraine, Russia's independent Levada Centre polling institute says, with more than 80 percent of Russians saying they support his actions.

The first poll the centre has conducted since the conflict began shows 83 percent of Russians back their leader, up from 71 percent in early February.

- Biden taps oil reserves -

Biden announces an unprecedented release of crude from US strategic oil reserves, saying it will "ease the pain" of rising fuel prices for Americans.

- Foreign ministers meeting mooted -

Turkey says the foreign ministers of Russia and Ukraine could meet "within about a week or two" to step up negotiations on ending the five-week-old conflict.

- New US sanctions -

The United States hits a series of Russian tech firms with sanctions, including the nation's largest chip maker Mikron.

- Russia bans EU leaders - 

Russia says it will expand the list of EU figures banned from entering the country following Western sanctions.