The United States has sought to woo Europe and Japan with free trade deals to gain leverage in an escalating tariff war with China but its overtures faced stiff resistance from France at a G20 finance ministers meeting dominated by trade tensions.

US Treasury Secretary Steven Mnuchin told reporters at the gathering of the financial leaders of the world's 20 largest economies in Buenos Aires that he was renewing US President Donald Trump's proposal that G7 allies drop trade barriers between them.

"If Europe believes in free trade, we're ready to sign a free trade agreement," Mr Mnuchin said, adding that such a deal would require the elimination of tariffs, non-tariff barriers and subsidies. "It has to be all three issues."

Mr Trump has angered European allies by imposing import tariffs of 25% on steel and 10% on aluminum, causing the European Union to retaliate with similar amounts of tariffs on Harley-Davidson motorcycles, Kentucky bourbon and other products.

Mr Trump, who frequently criticises Europe's 10% car tariffs, is also studying adding a 25% levy on automotive imports, which would hit both Europe and Japan hard.

French Finance Minister Bruno Le Maire said the European Union would not consider launching trade talks with the US unless Mr Trump first withdraws the steel and aluminum tariffs and stands down on a car tariff threat.

"We refuse to negotiate with a gun to our head," Mr Le Maire told reporters on the sidelines of the G20 meeting.

Mr Mnuchin's offer to the EU and Japan - along with a renewed effort to jump-start stalled talks with Mexico and Canada to modernise the North American Free Trade Agreement - come as the US and China are at loggerheads in an escalating trade dispute with no talks in sight.

The world's two largest economies have slapped tariffs on $34 billion worth of each other's goods so far.

Mr Trump has threatened to impose tariffs on all $500bn of Chinese exports to the US unless Beijing agrees to major structural changes to its technology transfer, industrial subsidy and joint venture policies.

Mr Mnuchin is not meeting formally with any Chinese officials at the G20 meeting, but said that was because his normal counterpart, top Chinese economic adviser Liu He, is not attending.

The International Monetary Fund also warned that the recent wave of trade tariffs would significantly harm global growth.

IMF Managing Director Christine Lagarde presented the G20 finance ministers and central bank governors meeting in Buenos Aires with a report warning that existing trade restrictions would reduce global output by 0.5%.

In the briefing note prepared for G20 ministers, the IMF said global economic growth may peak at 3.9% in 2018 and 2019, while downside risks have increased due to the growing trade conflict.

Ms Lagarde's presentation came shortly after Me Mnuchin said there was no "macro" effect yet on the US economy.

Mr Mnuchin said that, while there were some "micro" effect ssuch as retaliation against US-produced soybeans, lobsters and bourbon, he did not believe that tariffs would keep the US from achieving sustained 3% growth this year.

"I still think from a macro basis we do not see any impact on what's very positive growth," Mr Mnuchin said, adding that he is closely monitoring prices of steel, aluminum, timber and soybeans.