World leaders are gathering in Argentina for their annual G20 summit amid new tension with Russia over Ukraine and a US trade row with China.
US President Donald Trump has cancelled a meeting with his Russian counterpart, Vladimir Putin, in protest at Russia's seizure of Ukrainian naval boats.
Twenty-four sailors were arrested in the Black Sea incident.
A massive security operation is under way for the summit in the Argentine capital, Buenos Aires.
A bank holiday has been declared for Friday and the city's main business district has been shut down.
SUMMIT AGENDA
Hopes of progress over tariffs in Trump's expected talks with Chinese leader Xi Jinping have been dampened.
The two economic giants are locked in a trade war, which may possibly even escalate.
Climate change promises to be another major sticking point in the talks.
French President Emmanuel Macron was quoted by AFP news agency as saying he would refuse to sign a trade deal with South America's Mercosur bloc if Brazilian President-elect Jair Bolsonaro withdrew from the Paris climate accord.
The summit is also a diplomatic test for Saudi Crown Prince Mohammed bin Salman amid continuing questions about the Saudi state's possible involvement in the murder last month of journalist Jamal Khashoggi in Turkey.
because the Ukrainian ships and sailors seized by Russia in the Black Sea near Crimea had not been returned.
The decision also follows
during the 2016 election.
Putin's spokesman, Dmitry Peskov, said Moscow regretted the decision. But in his initial reaction to reports of the cancellation he said: "If this is so, the president will have a couple of extra hours in the programme for useful meetings on the sidelines of the summit."
German Chancellor Angela Merkel blamed the crisis "entirely" on Russia and said she would raise the issue with Putin.
However,
by a technical fault in her plane.
The plane was forced to land in Cologne on Thursday evening and she will not get to Buenos Aires until after other leaders have started their discussions.
President Trump said recently that
.
He also threatened tariffs on $267bn of other Chinese exports to the US.
Speaking to reporters at the White House, he said that while China was interested in striking a deal, "I don't know if I want to do it" and "I like the deal we have now".
The US has hit a total of $250bn of Chinese goods with tariffs since July, and China has retaliated by imposing duties on $110bn of US products.
"I think the most likely scenario is that [Chinese President] Xi Jinping doesn't offer big enough concessions to Trump, and so nothing much comes of the G20 meeting," said Julian Evans-Pritchard from Capital Economics.
International indignation over Khashoggi's murder, which was carried out inside Saudi Arabia's consulate in Istanbul on 2 October, continued on Thursday as
.
British Prime Minister Theresa May has said she will meet the crown prince and deliver a "very clear" message.
"In relation to Khashoggi, we want to see a full and transparent investigation of what happened and those responsible being held to account," she said.
WHAT IS G20
The Group of 20 brings together leaders of 19 of the world's most industrialised nations plus the EU.
Argentina, Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, the UK and the US are all represented.
, according to the event's website.
Last year's summit, in the German city of Hamburg, was marred by
.
In Buenos Aires, which hosts the first G20 meeting to be held in South America, more than 20,000 police have been deployed to maintain order.