(AFP) - UN Secretary-General Antonio Guterres on Thursday visited sites of alleged Russian war crimes in Ukraine, decrying war as "an absurdity in the 21st century" and urging Russia to cooperate with an international investigation into atrocities.

Making his first visit to Ukraine since Russia launched a full-scale invasion on February 24, Guterres toured several towns and villages outside Kyiv where Russian forces are accused of killing civilians.

"I imagine my family in one of those houses that is now destroyed and black. I see my granddaughters running away in panic," the UN chief said in Borodianka, a ruined town north-east of the Ukrainian capital.

"The war is an absurdity in the 21st century. The war is evil," he added.

In neighbouring Bucha, where dozens of bodies in civilian clothes, some with their hands tied behind their backs, were discovered this month after a Russian withdrawal, Guterres backed an International Criminal Court investigation into possible war crimes in Ukraine.

"I appeal to the Russian Federation to accept, to cooperate with the ICC," he implored the Kremlin.

The UN head will later meet Ukrainian President Volodymyr Zelensky. On Tuesday, he met Russian President Vladimir Putin in Moscow, repeating calls for both countries to work together to set up "safe and effective" humanitarian corridors in war-torn Ukraine.

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.

- 'One hope' -

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

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

With the war now into a third month and 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.

Ukraine's Defence Minister Oleksiy Reznikov warned of "extremely difficult weeks" as Moscow tries "to inflict as much pain as possible".

Senior presidential aide Mykhaylo Podolyak insisted Kyiv has the "right" to strike Russian military targets, suggesting direct attacks on facilities inside Russia.

The defence ministry in Moscow said its forces had destroyed two arms and ammunition depots in eastern and southern Ukraine overnight with "high-precision missiles".

It said its air force also targeted 67 Ukrainian military sites while air defence systems destroyed a Ukrainian fighter jet in the Lugansk region.

Russia has in recent days targeted Western-supplied arms, as the United States and Europe increasingly heed Zelensky's call for heavier firepower.

- 'Unacceptable threats' -

In a defiant speech Wednesday, Putin said if Western forces intervene in Ukraine and create "unacceptable threats", they will face a "lightning-fast" military response.

"We have all the tools for this, that no one else can boast of having," he told lawmakers, implicitly referring to Moscow's ballistic missiles and nuclear arsenal.

The Kremlin reiterated the warnings Thursday, saying Western arms deliveries "threaten" Europe's security.

Western allies remain wary of being drawn into war with Russia but have stepped up military support.

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

It drew praise from Podolyak, Zelensky's senior aide, as marking "the return of (German) leadership" in Europe.

The White House proposed using assets seized from Russian oligarchs to compensate Ukraine for damage caused by Moscow's invasion of the country, part of a US attempt to ratchet up economic punishment on the Kremlin.

President Joe Biden was to announce the proposed legislation alongside his request to increase funding by Congress for Ukraine's military later Thursday.

Meanwhile the civilian and military administrator of the Russian-controlled region of Kherson in southern Ukraine was quoted as saying that the ruble will soon be introduced in areas under Moscow's control.

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

- 'Blackmail' -

In its economic standoff with the West, Russia cut gas supplies to Bulgaria and Poland, both EU and NATO members, Wednesday.

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

Bulgaria and Poland are since receiving gas from EU neighbours, as Brussels warned it will not waver in its support for Kyiv, accusing the Kremlin of attempted "blackmail".

With the 27-member bloc, which has heavily depended on gas from Russia, scrambling to diversify its energy sources, European Commission President Ursula von der Leyen declared: "The era of Russian fossil fuels in Europe will come to an end".

European powers have imposed massive sanctions on Russia since Putin's invasion but have moved slowly on hitting Moscow's vast exports.

Last year, Russia supplied 32 percent of the total gas demand of the European Union and Britain, according to the International Energy Agency, although Europe's biggest economy, Germany, is particularly reliant on Russian energy.

Tensions have also risen in Transnistria, an unrecognised breakaway region of Moldova that borders southwestern Ukraine.

Authorities there have reported several explosions and incidents this week that it called "terrorist attacks", leading Kyiv to accuse Moscow of seeking to expand the war further into Europe.

"We are alarmed by the escalation of tensions in Transnistria," Russian foreign ministry spokeswoman Maria Zakharova said, saying Moscow expected "a thorough and objective investigation".

© Agence France-Presse