Russian forces bombarded Ukraine's capital late Thursday during a visit by UN Secretary-General Antonio Guterres, who decried the "absurdity" of war in the 21st century, as US President Joe Biden asked for $33 billion more to support Kyiv.

At least three people were wounded in the strikes in the western part of the city, which were the first in the capital in nearly two weeks, and came after Guterres toured Bucha and other suburbs where Moscow is alleged to have committed war crimes.

Ukrainian prosecutors said they were investigating 10 Russian soldiers for suspected atrocities in Bucha, where dozens of bodies in civilian clothes were found after Moscow's retreat, and had identified more than 8,000 alleged war crimes cases.

Those cases involve "killing civilians, bombing of civilian infrastructure, torture" and "sexual crimes" reported during Russia's occupation of various parts of Ukraine, prosecutor general Iryna Venediktova told a German TV channel.

In Washington, Biden urged US lawmakers to approve his huge aid package, as well as proposed new laws to allow using luxury assets stripped from Russian oligarchs to compensate Ukraine for damage inflicted since Moscow invaded on February 24.

"The cost of this fight is not cheap. But caving to aggression is going to be more costly if we allow it to happen," Biden said. 

Ukrainian President Volodymyr Zelensky hailed Biden's proposal as an "important step" and "necessary."

- 'Shocking' -

In a residential part of Kyiv, AFP correspondents saw one building in flames and black smoke pouring into the air.

"I heard the sound of two rockets and two explosions. It was a sound similar to a flying plane, and then two explosions with an interval of three to four seconds," Oleksandr Stroganov, 34, told AFP.

Mayor Vitali Klitschko said there had been "two hits in the Shevchenkovsky district", with one hitting "the lower floors of a residential building". Three people were taken to hospital, but the extent of their injuries was not immediately clear.

"It is a war zone but it is shocking that it happened close to us," Saviano Abreu, spokesman for the UN's humanitarian office who was travelling with Guterres, told AFP, adding that the delegation was safe.

Zelensky said the strikes, immediately after his talks with Guterres, were an attempt by Russia "to humiliate the UN and everything that the organisation represents."

A Russian military spokesman told reporters that Moscow's army had conducted air strikes against 38 military targets, destroying seven munitions depots, but made no immediate mention of Kyiv or the wider region around the capital.

Guterres, who arrived in Kyiv after talks in Moscow with Russian President Vladimir Putin, called the war "evil" after visiting Bucha and demanded that the Kremlin cooperate with an International Criminal Court investigation into the accusations.

Prosecutors said the 10 servicemen under investigation are suspected of "premeditated murder", cruel treatment and other violations of the laws and customs of war during their occupation of Bucha, northwest of Kyiv, in March.

At a joint press conference with Zelensky, Guterres admitted the UN Security Council had "failed to do everything in its power to prevent and end this war."

"This is the source of great disappointment, frustration and anger", he added.

- 'Idle comments' -

In Washington, Biden pushed back against increasingly heated claims by Russian officials and state media that Moscow is fighting the entire West, rather than only Ukraine.

"We're not attacking Russia. We are helping Ukraine defend itself against Russian aggression," Biden said.

The US leader also blasted Moscow for "idle comments" on the possible use of nuclear weapons.

The $33 billion package sought by Biden is a significant increase on previous requests.

The bulk -- $20 billion -- would be weapons and other security assistance, while $8.5 billion will be economic aid to help Ukraine's government respond to the crisis.

Russia has targeted Western-supplied arms, as the United States and Europe increasingly heed Zelensky's call for heavier firepower, despite their misgivings about being drawn into a conflict with Moscow.

The German parliament on Thursday voted overwhelmingly in favour of providing Kyiv with heavy weapons, a major shift in policy.

In its economic standoff with the West, Russia on Wednesday cut gas supplies to Bulgaria and Poland, both European Union and NATO members. Both are now receiving gas from EU neighbours.

On Thursday, Bulgaria's Prime Minister Kiril Petkov urged Europe to be "stronger" and wean itself off Russian gas as he also visited Ukraine, arguing "everybody in Europe should be able to".

Biden said the West would not allow Russia "to use their oil and gas to avoid consequences for their aggression".

- 'One hope' -

With the war claiming thousands of lives, Kyiv has admitted Russian forces are making gains in the east, capturing a string of villages in the Donbas region.

The first phase of Russia's invasion failed to reach Kyiv or overthrow Zelensky's government after encountering stiff Ukrainian resistance reinforced with Western weapons.

The Russian campaign has since refocused on seizing the east and south of the country while using long-range missiles against west and central Ukraine.

Guterres said the United Nations was doing everything possible to evacuate civilians from the "apocalypse" in the besieged port city of Mariupol.

A Russian official meanwhile said Thursday that the ruble will soon be introduced in areas of Ukraine under Moscow's control, despite Russia earlier insisting it was not seeking to occupy captured territory.

Ukraine's ombudsman condemned the move as "act of annexation" and "gross violation" of UN Charter articles.

Nearly 5.4 million Ukrainians have fled their country since the invasion, according to the United Nations, and more than 12 million others are displaced internally.

"We feel bad, we shouldn't be standing here," Svitlana Gordienko, a nurse forced to relocate to the southern city of Zaporizhzhia, told AFP as she queued for food at a humanitarian hub.

"We're left with only one hope: to return home," added pensioner Galina Bodnya.

Latest developments

Here are the latest developments in the war in Ukraine:

- 10 suspects in Bucha inquiry -

Ukrainian prosecutors say they have identified 10 Russian soldiers suspected of committing war crimes in the Kyiv suburb of Bucha.

The troops from the 64th motorised infantry brigade are suspected of premeditated murder, cruel treatment and other violations during their occupation of Bucha in March.

Investigators have also identified more than 8,000 cases of suspected war crimes since Russia's invasion.

- Security Council 'failed' -

Guterres says during his visit to Kyiv the Security Council failed to go far enough in its efforts to "prevent and end" Russia's war in Ukraine. 

"Let me be very clear: the Security Council failed to do everything in its power to prevent and end this war," he tells a news conference with Zelensky.

"And this is the source of great disappointment, frustration and anger."

- Kremlin warning on weapons -

The Kremlin says Western arms deliveries to Ukraine are a threat to European security.

"The tendency to pump weapons, including heavy weapons into Ukraine, these are the actions that threaten the security of the continent, provoke instability," Kremlin spokesman Dmitry Peskov tells reporters.

Britain's Foreign Secretary Liz Truss earlier called on Kyiv's allies to send more tanks and other heavy weapons. 

The German parliament meanwhile backs providing Ukraine with heavy weapons, marking a shift from its previously cautious approach.

- OSCE ends mission -

The world's largest security body, the OSCE, formally announces the end of its eight-year mission in Ukraine.

The body was the only international organisation monitoring the conflict in eastern Ukraine, but was forced out after Russia vetoed an extension of its mission last month.

- Ukrainian depots destroyed -

Russia's defence ministry says its forces have destroyed seven Ukrainian fuel and ammunition depots with "high-precision missiles".

- Briton killed -

The UK's foreign ministry confirms to AFP that a British citizen has been killed in Ukraine and another is missing.

British media report that the pair may have been fighting Russian forces. Several British and dual British-Ukrainian citizens are known to have volunteered on the Ukrainian side.