Russian President Vladimir Putin denounced the "vicious" use of sanctions and trade protectionism, in a veiled swipe at United States President Donald Trump at a G20 summit.

"One cannot help but see a dishonest competition increasingly taking the place of honest dialogue based on equality among states," Putin told leaders of emerging economies as the summit opened in Buenos Aires.

"A vicious practice of returning to illegal, unilateral sanctions and protectionist measures is spreading, going around the UN Charter, the rules of the WTO and internationally recognised legal norms," Putin said.

Putin said that the result was "an extremely negative effect on the spirit of international cooperation," discouraging business.

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Trump, who has advocated an "America First" policy on trade, has pledged to snub Putin in Buenos Aires in protest at Russia's naval skirmish with Ukraine in waters near Moscow-annexed Crimea.

Trump's decision comes as an investigation mounts at home into whether his presidential campaign colluded with Russia in the 2016 election and as Congress presses for tougher sanctions against Moscow.

Trio at loggerheads sign new trade deal

US President Trump, Canadian Prime Minister Justin Trudeau and Mexican President Enrique Peña Nieto signed a new US-Mexico-Canada Agreement — or USMCA — in Buenos Aires.

The trio, who have often been at loggerheads on trade, used the G-20 Summit to resolve the trade dispute with America's closest neighbours.

Trump spent more than a year pushing the leaders of Canada and Mexico into agreeing to a rewrite of North American trade rules.

"This has been a battle, and battles sometimes make great friendships," Trump said at the start of the signing ceremony.

Saying that all three countries will benefit from the deal, Trump said of the USMCA, "It is probably the largest trade deal ever made."

Trump now faces what could prove a more formidable foe: the US Congress, which must ratify the agreement.

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He says the parties have "taken a lot of barbs and a little abuse" during the renegotiation.

Saudi Crown Prince meets with G20 leaders

Saudi Crown Prince Mohammed bin Salman has talked with world leaders at a Group of 20 summit clouded by concerns about the gruesome killing of Saudi journalist Jamal Khashoggi.

Journalists saw the crown prince speaking with French President Emmanuel Macron and others at the summit venue on Friday in Buenos Aires.

Macron's office wouldn't comment on what they discussed.

On the eve of the summit, the French president called fors international involvement in the investigations into Khashoggi's killing in the Saudi Embassy in Istanbul last month.

EU leaders urged G20 participants to address concerns about journalist safety and the Saudi-backed war in Yemen.

US intelligence agencies have concluded the crown prince ordered Khashoggi's killing. Saudi Arabia has denied that.

Trump has said he won't be meeting with the Crown Prince at the G20.