World leaders are meeting 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.
But ahead of the summit's official start Mr Trump signed a trade deal with the Mexican and Canadian leaders.
A massive security operation is under way for the summit in Buenos Aires.
A bank holiday has been declared for Friday and the city's main business district has been shut down.
Speaking before the signing of the US-Mexico-Canada Agreement (USMCA) - to replace the Nafta free trade deal - Mr Trump described it as "probably the greatest trade deal ever".
"All of our countries will benefit greatly," he said.
What is going to be talked about at the summit?
Hopes of progress over tariffs in Mr 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.
What has been said about the Ukraine crisis?
Mr Trump said he would not meet Mr Putin because the three Ukrainian vessels and 24 sailors seized by Russia in the Black Sea near Crimea had not been returned.
The decision also follows the US president's former lawyer Michael Cohen pleading guilty to lying to Congress about a Trump property deal in Russiaduring the 2016 election.
Mr 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 Mr Putin.
However, Mrs Merkel's own arrival at the summit has been delayed by a technical fault in her plane.
The plane was forced to land in Cologne on Thursday evening. She has resumed her journey but will not get to Buenos Aires until after other leaders have started their discussions.
How intense is the trade war?
President Trump said recently that current tariff levels on $200bn (£157bn) of Chinese imports would rise as planned.
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".
Chinese Foreign Ministry spokesman Geng Zhuang said he was hopeful.
"We hope the US can show sincerity and meet China halfway, to promote a proposal that both countries can accept," he said, quoted by Reuters.
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.
What can Crown Prince Mohammed expect?
International indignation over Khashoggi's murder, which was carried out inside Saudi Arabia's consulate in Istanbul on 2 October, continued on Thursday as Canada slapped sanctions on 17 Saudi nationals allegedly linked to his murder.
British Prime Minister Theresa May has said she will meet the crown prince and deliver a "very clear" message.
"In relation to Mr Khashoggi, we want to see a full and transparent investigation of what happened and those responsible being held to account," she said.
Just what is the 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.
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.
The focus of the two-day summit opening on Friday is "fair and sustainable development", according to the event's website.
Last year's summit, in the German city of Hamburg, was marred by violent protests.
Have your say
More World Headlines
- UN backs end to UK control of Chagos islands
- Armed gang kills at least 18 in northwest Nigeria
- Female BBC manager rejects job over equal pay
- U.S. House's Pelosi: Trump is engaged in 'cover-up'
- Uber driver was Somali war criminal
- A baby cut from his mother's womb after she was killed opens his eyes
- South Sudanese journo beats Dangote in Fortune magazine’s World’s Greatest Leaders list
- Vibrator prize for Spanish squash champion spurs sexism debate
- White House tells ex-aide not to testify
- 12 Jihadists killed as Egypt investigates bomb explosion
- Kenya warns foreigners running illegal betting firms
- The Indian Dalit man killed for eating in front of upper-caste men
- Boat carrying Uganda footballers capsizes
- Saudi Arabia gives Sudan junta $250m
- Boat packed with footballers capsizes in Lake Albert