An expected high-profile bilateral meeting between Mr Trump and Russia's Vladimir Putin planned for Saturday was abruptly cancelled by Mr Trump, who made the announcement in a tweet citing Russia's seizure of Ukrainian vessels over the weekend.

Protesters have vowed a mass rally later Friday, underlining security concerns after recent football violence forced the relocation of a showpiece final to Spain.

After the photo was taken, the crown prince did manage a hearty exchange with Mr Putin.

May told British media she meant to use the summit to sell the vision of a "global Britain" after its Brexit departure from the European Union, scheduled for March next year. Trump wrote on Twitter.

Mr Trump did not specify in his remarks, or privately to Mr Morrison, which "things" had most captured his attention, but the Prime Minister later ran through a checklist of measures, including his Government's commitment to trade, extra engagement in the Pacific region and a review of the Iran nuclear deal.

Investigator Robert Mueller revealed that former top Trump aide Michael Cohen had admitted direct communications with Putin's office.

With markets watching nervously, Trump has described the Xi encounter as a deadline for China to meet his demands or risk even further pressure.

Trump has thrown out the traditional U.S. playbook on free trade since his shock 2016 election, vowing to protect forgotten workers and demanding a renegotiation of the North American Free Trade Agreement with Canada and Mexico. But enough has been tweaked for Trump to declare victory on behalf of the U.S. workers he claims were cheated by NAFTA.

European Council President Donald Tusk urged G-20 leaders to discuss "trade wars, the tragic situation in Syria and Yemen and Russian aggression in Ukraine".

Washington and Beijing have imposed tariffs on hundreds of billions of dollars on each other's imports after Trump began an effort to correct what he views as China's unfair commercial practices.

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.

Russian Federation and Ukraine have traded blame over the weekend ship incident - which Mr Trump cited as a reason for cancelling a much-anticipated meeting with Mr Putin at the G20.

German Chancellor Angela Merkel was supposed to get in to Buenos Aires early Friday, but her arrival was delayed when her plane suffered a technical problem and returned to Germany on Thursday night.

Although the new pact inherits features from the old one, Trump has declared it a victory for the U.S. workers he claims were cheated by NAFTA and on Friday called it the most "modern and balanced agreement in history".

Global politicians in attendance include British Prime Minister Theresa May, French President Emmanuel Macron, Russia's Vladimir Putin, US President Donald Trump and Saudi Crown Prince Mohammed bin Salman.

It is the prince's first significant appearance overseas since the killing.

Facing the prospect of a no-statement summit, European delegations were trying to create a common front and may come out with their own separate declaration if they can't get the USA or others on board.

Trump has made a decision to pull the United States from the Paris accord on curbing carbon emissions, despite mounting warnings from scientists leading up to a UN climate summit starting next week in Poland. His opposition to collective action stands in defiance of scientists' increasingly urgent warnings that policy action is desperately needed to counter the climate threat.

President 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.

United Nations chief Antonio Guterres said in Buenos Aires that "this is a make-it-or-break-it moment" on the issue.

The G20 summit of the most powerful countries in the world opened in the Argentine capital Buenos Aires with a call for dialogue and consensus from Mauricio Macri, the president of the country hosting the event.

Argentines are grappling with soaring inflation and unemployment.

"There's a lot of people who don't have houses and don't have work". Only when they were about to leave did Argentine officials arrive to greet them.

The government is vowing zero tolerance of violence as it hosts its biggest ever global gathering.