Buenos Aires, Nov 28 (AFP) G20 leaders, wrestling with the US-China trade war and ascendant populism at their 10th summit this week, face a new crisis following Russia's naval confrontation with Ukraine.

In November 2008, countries representing four fifths of the world's economic output came together to forge a collective front against the financial panic then engulfing the West.

A decade on, that unity has vanished as US President Donald Trump shreds the consensus underpinning international trade.

Other G20 countries such as Brazil, Italy and Mexico have turned to populist leaders, or, in Britain's case, are grappling with a fractious electorate's decision to file for divorce from the European Union.

And this year's G20 host Argentina is enduring its own economic crisis, necessitating the deployment of more than 22,000 police against the threat of mass protests when the heads of government convene on Friday and Saturday.

"I would say that the G20 has lost a lot of the steam that it once had in the global financial crisis in 2008, 2009," Matthew Goodman, senior vice president at the Center for Strategic and International Studies in Washington, said during a briefing previewing the summit.

"The last summits have been somewhat disappointing and marked by contention, particularly over trade-related issues. And I would expect that to be the case again."

Trump's "America First" stance has already upended other recent world summits such as the G7 and the Asia-Pacific bloc APEC.

For the G20 as a whole, Argentine Foreign Minister Jorge Faurie told AFP that the summit should stress the importance of trade itself.

But given the dysfunction at those other summits, the G20 leaders are expected to stop short of repeating their vows to fight protectionism, which were a founding principle in 2008.

Given US objections, analysts also dismiss any notion that the group can replicate its past consensus on the need to confront climate change, just ahead of a UN summit on the planetary threat in Poland.

After imposing a raft of tariffs on Chinese goods and threatening more to come, the US president is due to meet with Chinese counterpart Xi Jinping for a working dinner in Buenos Aires.

His economic advisor Larry Kudlow told reporters on Tuesday that "the president said there is a good possibility that we can make a deal and he is open to it."

But he warned that "if things don't work out in this US-China summit meeting, he will invoke another $267 some odd billion dollars in tariffs." Trump himself told The Wall Street Journal that it was "highly unlikely" that he would accept China's request to defer the next increase scheduled for January 1.

The White House confirmed Trump will sit down with President Vladimir Putin for their first face-to-face meeting since a July summit in Finland, when he astonished observers by praising the Russian leader.

That bilateral has taken on new urgency after Russian vessels seized three Ukrainian ships near Crimea at the weekend, igniting a storm of protest in Europe.

However, Trump told The Washington Post he was waiting for a report on the incident from his national security advisors before making up his mind over whether he would meet Putin.

"That will be very determinative. Maybe I won't have the meeting," he said.

On Monday, his UN ambassador Nikki Haley condemned such "outlaw actions" by Russia.

The exact location of the Ukrainian ships when they were boarded is subject to dispute. Kiev, backed by allies in Western Europe, says they were in international waters in the Sea of Azov.

In the G20 context, that parallels controversies pitting Beijing against other nations over the passage of ships through the South China Sea, a vital trading artery.

"What's happening in the Sea of Azov is serious," said Francois Heisbourg, special adviser at the Foundation for Strategic Research in Paris.

"If you start transgressing on the principles of freedom of navigation, that could serve as a precedent around the world. Any country controlling a strait could be tempted to forbid passage." On the trade front, Heisbourg said the question at the G20 was whether Xi will "rear up" against Trump's combative stance, or try to find an accommodation between the world's two largest economies.

Analysts are skeptical that Buenos Aires will witness a breakthrough big enough to forestall Trump's threat to toughen the China tariffs in January -- or even if there is, whether it will hold for long.

Cui Lei, an expert at the Department for American Studies at Beijing's China Institute of International Studies, said the signs were not promising following the stormy APEC summit and new US accusations of Chinese sharp practice on fronts such as intellectual property rights and technology transfers.

"There may be a statement of principles (in Buenos Aires), confirming they will maintain negotiation and communication, but I think it will be difficult to reach a comprehensive agreement, because to do that both sides need to make big concessions," he said.

Markets will be watching the G20 for any steps by Saudi Arabia and Russia -- the world's two biggest exporters of crude -- to reverse a recent collapse in oil prices.

Such petro-diplomacy would, however, run headlong into Trump's loudly stated desire to keep oil prices low.

And Saudi Crown Prince Mohammed bin Salman can ill afford to alienate the one Western leader who seems to be standing by him following the murder of journalist Jamal Khashoggi in Turkey last month. (AFP) AMS

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