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.
Hopes of progress over tariffs in Mr Trump’s expected talks with Chinese leader Xi Jinping have been dampened.
The trade war between the two economic giants may possibly even escalate.
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.
Meanwhile, a massive security operation is under way for the summit in the Argentine capital, Buenos Aires, where a bank holiday has been declared for Friday and the city’s main business district has been shut down.
Russia captured three Ukrainian naval vessels and 24 personnel in the Black Sea this week after accusing them of trespassing.
Mr Trump said he would not meet the Russian leader “based on the fact that the ships and sailors have 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 Russia during the 2016 election.
Mr Putin’s spokesman, Dmitry Peskov, responded: “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.
Mrs Merkel’s own arrival at the summit has been delayed by a technical fault in her plane.
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.”
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 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.
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.
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.