Ukrainian forces are preparing for new Russian attacks in the east of the country as Moscow deploys more troops there after suffering setbacks near the capital Kyiv, President Volodymyr Zelenskiy said on Thursday.

Russia's invasion of its neighbour, now in its fifth week, has driven more than 3.5 million people from their homes, killed thousands, sent global oil prices soaring and brought Russian-Western tensions to their worst point since the Cold War.

The United States is considering a massive release of oil reserves to counter rising oil prices which are fuelling inflationary fears around the world. The International Energy Agency (IAE) will hold an emergency meeting on Friday.

Tough resistance by Ukrainian forces has prevented Russia from capturing any major city, including Kyiv, where a Russian armed column was held back for weeks.

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The head of Britain's GCHQ spy service said new intelligence showed some Russian soldiers had refused to carry out orders, sabotaged their own equipment and accidentally shot down one of their own aircraft.

In an early morning video address, Zelenskiy referred to Russian troop movements away from Kyiv and Chernihiv and said that was not a withdrawal but rather "the consequence of our defenders' work."

Zelenskiy added that Ukraine is seeing "a build-up of Russian forces for new strikes on the Donbas and we are preparing for that."

Russia says its forces are regrouping to focus on "liberating" the breakaway eastern Donbas region.

The Donbas region encompasses two self-proclaimed "people's republics" that Russia says it is helping to free from Ukrainian control.

The leader of the Donetsk People's Republic, Denis Pushilin, said on Wednesday that offensive operations were intensifying.

Donetsk includes the besieged port city of Mariupol, which has seen some of the war's heaviest fighting and bombardment and where about 170,000 people are trapped with scarce food and water.

"We cook what we find among neighbours. A bit of cabbage, a bit more of potatoes, we've found tomato paste, some beetroot," said former steel worker Viktor from Mariupol. They cook using a rudimentary barbecue and sleep in a basement, which he termed their "peaceful oasis."

Russian forces have taken half of the strategic port city, an adviser to Zelenskiy said on Wednesday. Russia's defence ministry said it was prepared to observe a ceasefire in Mariupol on Thursday, Russian news agencies reported.

Kyiv has accused Russia of not fully respecting earlier such commitments. Moscow denies targeting civilians.

Peace talks, Gas prices

Russia says it is carrying out a "special operation" to disarm and "denazify" its neighbour. Western countries say Moscow's invasion, the biggest assault on a European country since World War Two, was entirely unprovoked.

Peace talks will resume online on Friday but neither Ukraine nor Russia expect a quick resolution.

Ukraine has sought a ceasefire without compromising on territory or sovereignty, though it has proposed adopting a neutral status in exchange for security guarantees. Russia opposes Ukraine joining the US-led NATO military alliance, and has cited its potential membership as a reason for the invasion.

Western sanctions imposed on Russia as punishment for its invasion have largely isolated its economy from world trade but Moscow is still the biggest supplier of oil and gas to Europe and the war has seen global oil prices spike.

The United States, the world's largest crude producer, is considering releasing up to 180 million barrels of oil over several months from its Strategic Petroleum Reserves (SPR) to ease upward pressure on prices, two US sources said.

Oil prices dived more than $5 a barrel on Thursday on the news.

The move would mark the third time the United States has tapped its strategic reserves in the past six months, and would be the largest release in the near 50-year history of the SPR.

US President Joe Biden will give remarks later on Thursday on efforts to lower gas prices, the White House said.

International Energy Agency (IEA) member countries are set to hold an emergency meeting on Friday at 1200 GMT to decide on a collective oil release, a spokesperson for New Zealand's energy minister said on Thursday.

"The amount of the potential collective release has not been decided. That meeting will set a total volume, and per country allocations will follow," the spokesperson for energy minister Megan Woods said.

Seeking to exert its leverage, Russia demanded oil and gas payments be made in roubles by Friday, raising fears of energy shortages and boosting recessionary risks in Europe. Germany has warned of a possible emergency if Russia cuts supplies.

Floating a potential compromise, Russian President Vladimir Putin told German Chancellor Olaf Scholz that payments could be made in euros and sent to Gazprombank, which would convert the money to roubles, a German government spokesperson said.

Scholz asked for written information to "better understand the procedure," the spokesperson said.

Gazprombank, one of the main channels for payments for Russian oil and gas, is not subject to EU sanctions.