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.