US President Joe Biden made an unannounced visit on Monday to Ukraine to meet with President Volodymyr Zelensky, a gesture of solidarity that comes days before the one-year anniversary of the Russian attack.

Biden met with Zelensky at Mariinsky Palace in Kyiv, announcing an additional US$500 million in US assistance and reassuring Ukraine of American and allied support as the conflict continues.

“One year later, Kyiv stands. And Ukraine stands. Democracy stands,” Biden said.

"Your visit is an extremely important sign of support for all Ukrainians," Zelensky said.

Air raid sirens blared across the Ukrainian capital as Biden visited Kyiv but there were no reports of Russian missile or air strikes.

The visit by Biden to Ukraine comes as China's top diplomat, Wang Yi, prepared for a visit to Moscow, and possibly a meeting with Russian President Vladimir Putin, to discuss ideas for peace in Ukraine.

Kremlin spokesman Dmitry Peskov confirmed the planned visit by Wang to Moscow but gave no date for the trip.

"We don't rule out a meeting between Mr Wang and the president (Putin)," Peskov told reporters. "The agenda is clear and very extensive, so there is lots to talk about." (Agencies)