Beijing has throughout refused to condemn Moscow's invasion, now into its fifth week, as it treads a diplomatic tightrope between backing its close ally and maintaining ties with the West.

But when asked about the reported discovery of dozens of bodies in mass graves or littering the streets in Bucha over the weekend, China said the "reports and images of civilian deaths in Bucha are deeply disturbing."

Foreign ministry spokesman Zhao Lijian also said any "accusations should be based on facts" as he said any humanitarian situation should not be "politicised".

"Before the investigation results are out, all sides should maintain restraint and avoid baseless accusations," he told reporters at a regular press briefing.

The Ukrainian army retook control of the key commuter town of Bucha outside Kyiv just a few days ago and said it had found dozens of bodies after Russian forces pulled out.

The reported events in Bucha have sparked global outrage, including from the Pope and the US, which has called for a "war crimes trial" over the alleged atrocities.

The Kremlin has denied the killings and claimed the images emerging from Bucha are fakes produced by Ukraine.

Zhao added Wednesday that China "pays very close attention to the harm suffered by civilians", and is "willing to continue working with the international community to avoid any civilian harm."

His statement echoed earlier remarks by China's ambassador to the UN, Zhang Jun, at a Tuesday meeting of the Security Council.

At the same session Ukraine's President Volodymyr Zelensky condemned the killings in the town reclaimed from Russian troops as a "genocide" and urged the UN Security Council to expel Russia as a permanent member.

Zelensky called on the 15-member council -- which aims to ensure international peace and security -- to expel Russia "so it cannot block decisions about its own aggression, its own war."

"If there is no alternative and no option, then the next option would be to dissolve yourself altogether," Zelensky continued.

China has so far provided humanitarian aid worth around $2.3 million to Ukraine, an amount dwarfed by the contributions of many other countries.