Buenos Aires — US farmers would receive the first payments of a $12bn subsidy programme designed to shield them from a burgeoning trade war starting in September, Agriculture Secretary Sonny Perdue said.

"The mitigation payments will be eligible in September in some degree and into October" and autumn, Perdue said. "It depends on when the farmer comes in and applies."

US President Donald Trump announced the subsidy last week as he sought to ease anxiety among farmers who feel they will be shut out of important markets in Asia.

Trump has applied tariffs to steel and aluminium imports, threatened to place tariffs on vehicles made in the EU and said he would place levies on $500bn of Chinese imports.

All these measures have heightened tension that a burgeoning trade war may damp global growth.

The US soya bean harvest is from September to November.

Money for the payments will be tapped from programmes already authorised by the US Department of Agriculture, Perdue said. The subsidies would be for the 2018 crop season only because farmers could not have foreseen current global trade feuding when they planted.

In 2019, he added, growers would not need the help as they will adapt their sowing plans to market conditions.