US President Joe Biden and other G20 leaders gathered in New Delhi on Friday for their annual summit, as deep divisions between heavyweight members and a no-show by China's Xi Jinping called the bloc's relevance into question.

The Group of 20 was conceived in the throes of the 2008 financial crisis as a way of managing the global economy.

But finding consensus among members has been increasingly difficult in recent years.

With India's capital spruced up and partly emptied of people for the occasion, the host scrambled for last-minute agreement on vexed issues including the Ukraine war, climate and global governance.

United Nations Secretary-General Antonio Guterres warned leaders that their ongoing squabbles risked stoking conflict and corroding public trust.

"If we are indeed one global family -- we today resemble a rather dysfunctional one", Guterres told reporters in New Delhi.

Even before it began, the importance of the G20 summit was called into question when China's Xi decided to skip the meeting and send his number two, Premier Li Qiang instead.

No official reason was given for Xi's absence, but the Asian giants have been at loggerheads over a border dispute and other issues, while Beijing is seeking to make US-led groupings such as the G20 more amenable to its own interests.

Guterres insisted countries must assume responsibility regardless of "whether it's the president or the prime minister or the vice president that comes" to New Delhi.

But a failure of G20 leaders to agree a joint summit statement, a once routine diplomatic affair, would be a major embarrassment for the host.

Indian Prime Minister Narendra Modi has painted the summit as his country's diplomatic coming of age -- evidence of New Delhi's clout and prestige on the global stage.

The Indian leader, sensing an opportunity to burnish his credentials as a statesman ahead of a re-election tilt early next year, has also thrust himself front and centre of proceedings.

His image adorns countless G20 billboards and posters plastered around the capital.

- G20 to G21? -

Modi does look set to secure one notable diplomatic victory, with several leaders expressing support for expanding the bloc into the "G21" by including the African Union as a permanent member.

An invitation to join could come as soon as Saturday.

But on Ukraine there was little sign of progress.

Diplomatic opprobrium and war crimes charges have kept Russian leader Vladimir Putin from the summit.

But Moscow continues to press allies to water down international condemnation of its invasion of Ukraine, throwing up a major roadblock to joint action.

British Prime Minister Rishi Sunak said Putin was "the architect of his own diplomatic exile".

"The rest of the G20, meanwhile, are demonstrating that we will turn up and work together to pick up the pieces of Putin's destruction."

Ahead of his arrival Biden insisted that G20 can still "deliver", even as markets fret that a trade war between the world's two largest economies is poised to escalate.

Rumours have swirled that Beijing may be about to ban Apple's ubiquitous iPhone -- most of which are made in China.

Many G20 leaders fear their economies are already at risk of becoming collateral damage as the big beasts of world trade lock horns.

Economists say US restrictions on the transfer of sensitive technologies to China have deepened a slowdown in the world's second-largest economy.

They also point to serious structural problems in China -- overtaken as the world's most populous country by India earlier this year -- such as a shrinking labour force, slower productivity and an overheated real estate market.

Speaking in New Delhi on Friday, US Treasury Secretary Janet Yellen warned a Chinese slowdown carried risks for the entire globe.

"China faces a variety of both short- and longer-term global challenges, economic challenges that we've been monitoring carefully," she said.

"That said, China has quite a bit of policy space to address these challenges."