Chinese President Xi Jinping and U.S. President Joe Biden defused Cold War rhetoric in long-awaited talks that come as relations between their countries hit their lowest in decades, marred by disagreements over a host of issues from Taiwan to trade.

The two, holding their first in-person talks since Biden became president, met on the Indonesian island of Bali ahead of a Group of 20 (G-20) summit Tuesday that is set to be fraught with tension over Russia's invasion of Ukraine.

"As the leaders of our two nations, we share responsibility, in my view, to show that China and the United States can manage our differences, prevent competition from... turning into conflict, and to find ways to work together on urgent global issues that require our mutual cooperation."

He mentioned climate change and food insecurity as problems the world expected their two countries to address.

Responding to Biden, Xi said the relationship between their two countries was not meeting global expectations.

"So we need to chart the right course for the China-U.S. relationship. We need to find the right direction for the bilateral relationship going forward and elevate the relationship," Xi said.

"The world expects that China and the United States will properly handle the relationship," he said, adding he looked forward to working with Biden to bring the relationship back on the right track.

Neither leader wore a mask to ward off COVID though members of their delegations did.

Both presidents tried to take some heat out of their simmering superpower rivalry, during the three-hour summit that found common ground on Ukraine but left little doubt that stark differences remain.

Biden emerged from the meeting proclaiming there need not be a new Cold War, as both leaders spoke of the desire to prevent high tensions from spilling over into conflict.

Xi told Biden that the two countries "share more, not less, common interests," according to a Chinese account of the meeting, sounding more conciliatory than the last three pandemic-filled years without face-to-face presidential meetings would suggest.

"The world expects that China and the United States will properly handle the relationship," Xi told him.

Trying to scotch the notion that China is bent on usurping the United States and remaking the world in its own authoritarian image, Xi reportedly said Beijing does not seek to challenge the United States or "change the existing international order".

On the pressing issue of Russia's war in Ukraine and President Vladimir Putin's veiled threats to use nuclear weapons, the pair agreed that nuclear war should not be fought and cannot be won, according to the White House.

They "underscored their opposition to the use or threat of use of nuclear weapons in Ukraine," the US added.

That common cause is likely to give Putin pause as he weighs how to turn the tide of a war that his regime's survival could hinge on.

But Biden and Xi's meeting was no kumbaya summit.

The two leaders notably clashed on the question of Taiwan's future.

Tensions have risen sharply over Taiwan, with China in August conducting major military exercises after a visit to the self-governing democracy, which it claims, by US House Speaker Nancy Pelosi.

Xi told Biden that Taiwan is the "first red line that must not be crossed in China-US relations," according to the Chinese foreign ministry statement.

Biden told Xi he opposed any changes on Taiwan -- after the US leader repeatedly indicated that Washington was ready to defend the island militarily.

And he raised US "objections" to China's "coercive and increasingly aggressive actions toward Taiwan, which undermine peace and stability across the Taiwan Strait and in the broader region, and jeopardise global prosperity," the White House said.

Biden also nudged China to rein in ally North Korea after a record-breaking spate of missile tests has raised fears that Pyongyang will soon carry out its seventh nuclear test, and said he was "confident China's not looking for North Korea to engage in further escalation".

In a sign of thawing ties, Biden announced that US Secretary of State Antony Blinken will visit China "to follow up on their discussions".

A senior State Department official, speaking on condition of anonymity, told AFP the two countries were working "to schedule a visit tentatively planned for early next year".

Xi's last in-person meeting with a US president was in 2019 with Donald Trump, who along with Biden identified China as a top international concern and the only potential challenger to US primacy on the world stage.

Although the meeting was the first time Xi and Biden have met as presidents, the pair have an unusually long history together.

By Biden's estimation, he spent 67 hours as vice president in person with Xi including on a 2011 trip to China aimed at better understanding China's then-leader-in-waiting, and a 2017 meeting in the final days of Barack Obama's administration.

On Tuesday, Xi will hold the first formal sitdown with an Australian leader since 2017, Prime Minister Anthony Albanese announced, following a concerted pressure campaign by Beijing against the close US ally.

He will also meet French President Emmanuel Macron, and Britain's new Prime Minister Rishi Sunak has said he also hopes to speak with the Chinese leader.

- Absent Putin -Though he is engaging Xi, Biden has refused since the invasion of Ukraine to deal directly with Putin, who is conspicuously absent from the Bali summit.

The Kremlin cited scheduling issues and has instead sent longtime foreign minister, Sergei Lavrov, who arrived Sunday evening and underwent two health checks at a Bali hospital, according to an Indonesian health ministry official.

Lavrov, 72, denied reports that he was receiving treatment at a Bali hospital, telling Tass news agency that he was in his hotel preparing for the summit.

Lavrov's presence has thrown into question a customary G20 group photo and joint statement, with Russia sure to reject any explicit calls to end its invasion of Ukraine.