Russian President Vladimir Putin on Friday denounced the "vicious" use of sanctions and trade protectionism, in a veiled swipe at 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 recognized legal norms," Putin said.

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

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.

Earlier, in a move applauded in Kiev, U.S. President Donald Trump called off a meeting with Russia's Vladimir Putin in Argentina to signal Washington's disapproval of Russian behaviour in the naval clash with Ukraine.

The Russian rouble, which is sensitive to events that might lead to new sanctions being imposed on Russia, fell on news of the cancelled meeting.

In a further boost to Ukraine, the EU released 500 million euros in financial assistance to Kiev and European Council President Donald Tusk predicted Brussels would roll over sanctions on Russia at a summit on Dec. 13-14.