Ukrainian President Volodymyr Zelensky revealed on Thursday the discovery of a mass grave in the recently recaptured town of Izyum, likening it to deaths seen in the cities of Bucha and Mariupol, which have become symbols of Russian atrocities.

Hours later, the United States announced another $600 million in military aid to support Kyiv's two-week-old counteroffensive aimed at dislodging Russian occupiers.

"We want the world to know what the Russian occupation has caused," Zelensky said, without giving details on the number of bodies found or their cause of death.

Presidential chief of staff Andriy Yermak accused Russia of murder in a tweet about the mass grave, posting a photo of a forested area dotted with crudely made wooden crosses.

"All bodies will be exhumed and sent for forensic examination," he said. "Expect more information tomorrow."

A regional police official, Sergei Botvinov, told Sky News that a burial site with around 440 graves had been discovered in Izyum, with some of the dead shot and others killed in shelling.

"Also we have information that a lot of bodies have not been identified yet. So the reasons of death will be established during the investigations," he added.

The grim discovery came as the White House on Thursday announced it had approved a new package of up to $600 million in additional military aid for Ukraine, including equipment and services, as well as training.

Since Russia's invasion began, the United States has provided more than $15 billion in military assistance to Kyiv.

- 'As long as it takes' -

European Union chief Ursula von der Leyen was in Kyiv on Thursday for discussions with Ukraine's leaders about "getting our economies and people closer".

Her trip coincided with a meeting between Russian President Vladimir Putin and his increasingly close ally Chinese leader Xi Jinping in ex-Soviet Uzbekistan, where the men hailed their strategic ties in defiance of the West.

"We will never be able to match the sacrifice that the Ukrainians are making," von der Leyen told reporters during a joint press conference with Zelensky.

"But what we can tell you is that you'll have your European friends by your side as long as it takes."

Kyiv gained EU candidacy status in June at the same time as ex-Soviet Moldova, which borders Ukraine and like its neighbour has had Russian troops stationed in an eastern breakaway region.

The historic candidacy vote angered Moscow, which has tried to retain political and military influence in both countries since the collapse of the Soviet Union three decades ago.

EU countries have staunchly supported Ukraine since Moscow invaded in February by hitting Russia with economic penalties.

Many members of the bloc have also supplied Kyiv with advanced weapons that have helped Ukrainian forces in recent weeks recapture swathes of territory.

Germany's defence minister pledged more weapons on Thursday, saying Berlin would provide armoured vehicles and rocket launch systems -- but not the battle tanks sought by Ukraine.

Von der Leyen said just ahead of her trip that the successive waves of EU sanctions against Russia were "here to stay".

The Kremlin maintains that Russia has weathered the economic penalties. Moscow has responded by reducing or halting entirely gas flows to European countries.

- Putin praises Xi's stance -

Ukrainian energy facilities, including Zaporizhzhia -- Europe's biggest atomic plant -- have been hit by Russian strikes.

The UN nuclear agency on Thursday asked Russia to withdraw its troops from Zaporizhzhia, diplomats said.

Ukraine's forces have posted slow but steady gains in the southern Kherson region near the Black Sea.

The Ukrainian presidency said on Thursday that intense fighting was ongoing around that southern front, adding that the military situation "remains extremely difficult".

Local officials in the region around Zelensky's hometown of Kryvyi Rih reported fresh Russian strikes Thursday after attacks damaged a dam and saw dozens of homes flooded.

In the eastern Donetsk region, which has been partially controlled by Russian-backed separatists since 2014, fresh shelling killed two civilians and left another 13 wounded.

At the meeting in Uzbekistan, Putin blasted attempts to create a "unipolar world" and praised China's stance on the conflict.

"We highly appreciate the balanced position of our Chinese friends in connection with the Ukrainian crisis," Putin told Xi.

Beijing has not explicitly endorsed Moscow's invasion but it has steadily built economic and strategic ties with Russia over the six months of the war.