Meanwhile, Russia has welcomed Beijing taking a more active role in efforts to "resolve" the war after China's top diplomat Wang Yi met Vladimir Putin in Russia.

Following the meeting, Mr Putin said he was looking forward to a visit to Moscow by Chinese President Xi Jinping.

No other countries could influence the relationship between China and Russia, he added.

Russia's foreign ministry spokeswoman Maria Zakharova said Mr Wang's trip had shown Moscow and Beijing agreed on many issues.

Ms Zakharova praised China's "balanced approach" towards the war.

"We welcome China's readiness to play a positive role in resolving the Ukrainian crisis," Ms Zakharova said.

Mr Wang was quoted in Russian state media as saying China will "firmly adhere to an objective and impartial position and play a constructive role in the political settlement of the crisis".

China has refused to criticise the invasion of Ukraine while condemning sanctions imposed on Russia.

In return, Russia has supported China amid tensions with the US over Taiwan.

Russia is due to begin military exercises with China in South Africa on Friday and has sent a frigate equipped with new generation hypersonic cruise missiles.

A Russian officer said Moscow would fire artillery, but not the missiles, whose speed makes them difficult to shoot down.

Putin 'proud' of those fighting in Ukraine

The top Chinese diplomat's visit to Russia came as Mr Putin spoke at a huge rally in Moscow.

He said Russia is "proud of  those who are fighting in Ukraine to defend the fatherland", adding that the "whole country" supports them. 

Chants of "Russia, Russia, Russia" were heard around him as he spoke.

Some 200,000 people had gathered in Moscow to hear Mr Putin's address.

Biden criticises Russia's move to pause nuclear treaty cooperation

Meanwhile, Mr Biden said Mr Putin had made a "big mistake" in deciding to suspend participation in the last nuclear treaty between the US and Russia.

He made the comment to reporters as he arrived at the presidential palace in Warsaw for a summit of the Bucharest Nine countries.

Moscow has insisted its decision to pull out of the New START treaty does not raise the risk of nuclear war.

The move is expected to have an immediate impact on US visibility into Russian nuclear activities as it allowed each side to conduct up to 18 inspections of strategic nuclear weapons sites each year