Xi Jinping said Tuesday that China would prioritise ties with Russia, calling the two "great neighbouring powers" as he prepared for a second day of talks with Vladimir Putin expected to focus on Ukraine.

The Chinese president also said he invited the internationally isolated Putin to visit China later this year as both leaders seek an alliance to counteract Western power.

Beijing and Moscow's trade ties have boomed since Russia's Ukraine campaign, linking the nations more closely and raising worries in Western capital over how far the ties will go.

Xi said China's government would "continue to prioritise the all-round strategic partnership between China and Russia".

"We are great neighbouring powers," he was quoted by Russian news agencies as saying during a meeting with Prime Minister Mikhail Mishustin.

Xi's trip coincides with a surprise visit to Kyiv by Japanese Prime Minister Fumio Kishida, who arrived Tuesday in what Ukraine's First Deputy Foreign Minister Emine Dzheppar called a "historic" visit.

Writing on Twitter, she called it "a sign of solidarity and strong cooperation between (Ukraine and Japan). We are grateful to Japan for its strong support and contribution to our future victory."

- 'Constructive role' -

Xi's visit to Moscow has been viewed as a major boost for Putin, who is under Western sanctions and subject to an International Criminal Court warrant over accusations of unlawfully deporting Ukrainian children.

On Monday, Xi and Putin held four and a half hours of talks, calling each other "dear friend."

In a rare move, Putin escorted Xi to his car after the talks, and the two were seen smiling together.

During the meeting, the Russian leader said he was open to talks on Ukraine and praised Beijing's 12-point position paper on the conflict, which includes a call for dialogue and respect for all countries' territorial sovereignty.

Xi and Putin are also expected to discuss boosting economic cooperation as Russia boosts energy exports to China after being mostly shut out of European markets.

Ahead of the talks, Russian gas giant Gazprom said that supplies through the Power of Siberia pipeline to China had reached a daily record on Monday.

Xi's three-day visit began a day after Putin travelled to Mariupol in eastern Ukraine, his first trip to territory captured from Kyiv since the start of the assault in February 2022.

China has sought to portray itself as a neutral party in the Ukraine conflict, but Washington has said Beijing's moves could be a "stalling tactic" to help Moscow.

US Secretary of State Antony Blinken said Xi's Moscow visit "suggests that China feels no responsibility to hold the president accountable for the atrocities committed to Ukraine."

"And instead of even condemning, it would rather provide diplomatic cover for Russia to continue to commit those great crimes," he added.

The United States has accused Beijing of mulling arms exports to Moscow, claims China has vociferously denied.

Zelensky has said he would welcome talks with Xi, though there has been no indication from Beijing of any such plans.

- Kishida heads to Kyiv -

Meanwhile, Japanese leader Kishida was on his way to Kyiv, where he would offer "solidarity and support" in a meeting with Zelensky.

Kishida is the last Group of Seven leader to visit Ukraine and has come under increasing pressure to make the trip, as Japan hosts the grouping's summit this May.

Japan and China are close trading partners, but Tokyo has been increasingly worried about Beijing's growing assertiveness in the region.

Japan is part of the US-led security alliance known informally as the Quad, which also includes India and Australia, and positions itself as a bulwark against China's military ambitions in Asia and the Pacific.

Moscow and Beijing have over the past years ramped up cooperation, both driven by a desire to counterbalance US global dominance.

While Beijing has called for an "impartial" mediation in the conflict, Western countries have argued that China's proposals are heavy on grand principles but light on practical solutions.

The United States said last week that China's proposals would simply consolidate "Russian conquest" and allow the Kremlin to prepare a fresh offensive.

China and Russia have often worked in lockstep at the UN Security Council, using their veto power as permanent council members to counter the West.

It defended Putin on Monday against the International Criminal Court, saying that the court should avoid what it called "politicisation and double standards" and respect the principle of immunity for heads of state.

Russia's assault on Ukraine has also deepened fears among Western powers that China could one day try to take control of the self-ruled island of Taiwan, which Beijing sees as part of its territory.