By Nandita Bose and Fransiska Nangoy

NUSA DUA, Indonesia (Reuters) - Chinese leader Xi Jinping and U.S. President Joe Biden met on Monday for long-awaited talks that come as relations between their countries are at 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 (G20) summit on Tuesday that is set to be fraught with tension over Russia's invasion of Ukraine.

They smiled as they shook hands warmly in front of a row of Chinese and U.S. flags in a ballroom at the luxury hotel Mulia on Bali's Nusa Dua bay.

"We spent a lot of time together back in the day when we were both vice presidents and it's just great to see you," Biden told Xi as he put an arm around him, adding in remarks delivered in front of reporters that he was committed to keeping lines of communication open on a personal and government level.

"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 as leaders of two major countries, they needed to chart their course, find the right direction and elevate their relationship. Xi said 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.

Their main topics of discussion are expected to be Taiwan, Ukraine and North Korea's nuclear ambitions, issues that will also loom over the G20 that is being held without Russian President Vladimir Putin in attendance.

Russian Foreign Minister Sergei Lavrov will represent Putin at the G20 summit - the first since Russia invaded Ukraine in February - after the Kremlin said Putin was too busy to attend. Russia's foreign ministry said a report that Lavrov was taken to hospital after arriving in Bali was fake news.

On Sunday, Biden told Asian leaders in Cambodia that U.S. communication lines with China would stay open to prevent conflict, with tough talks almost certain in the days ahead.

Relations have been roiled in recent years by growing tensions over issues ranging from Hong Kong and Taiwan to the South China Sea, trade practices and U.S. restrictions on Chinese technology.

But U.S. officials said there have been quiet efforts by both Beijing and Washington over the past two months to repair ties.

"These meetings do not take place in isolation, they are part of a very sustained process," said one Biden administration official. "We have engaged in serious, sustained - dozens and dozens of hours - of quiet diplomacy behind the scenes.

U.S. Treasury Secretary Janet Yellen told reporters in Bali earlier that the meeting was "intended to stabilise the relationship between the United States and China, and to create a more certain atmosphere for U.S. businesses".

She said that Biden had been clear with China about national security concerns regarding restrictions on sensitive U.S. technologies and had raised concern about the reliability of Chine supply chains for commodities like minerals.

NUCLEAR 'IRRESPONSIBILITY'

Biden and Xi, who have held five phone or video calls since Biden became president in January 2021, last met in person during the Obama administration when Biden was vice president.

Monday's meeting is unlikely to produce a joint statement, the White House has said.

G20 summit host President Joko Widodo of Indonesia said he hoped the gathering could "deliver concrete partnerships that can help the world in its economic recovery".

However, one of the main topics at the G20 will be Russia's war in Ukraine and Biden will be "unapologetic" in his defence of the European nation, U.S. officials said last week.

Xi and Putin have grown increasingly close in recent years, bound by their shared distrust of the West, and reaffirmed their partnership just days before Russia invaded Ukraine. But China has been careful not to provide any direct material support that could trigger Western sanctions against it.

Chinese Premier Li Keqiang emphasised the "irresponsibility" of nuclear threats during the summit in Cambodia, suggesting China was uncomfortable with strategic partner Russia's nuclear rhetoric, the Biden administration official said.

The West has accused Russia of making irresponsible statements on the possible use of nuclear weapons since its February invasion of Ukraine. Russia has in turn accused the West of "provocative" nuclear rhetoric.

Russia's Lavrov said on Sunday the West was "militarising" Southeast Asia in a bid to contain Russian and Chinese interests, setting the stage for more confrontation with Western leaders at the G20.

Ukrainian President Volodymyr Zelenskiy has said he would address the G20 gathering by videolink on Tuesday.