"As @realDonaldTrump said, US sanctions will target critical sectors of Iran's economy.

"German companies doing business in Iran should wind down operations immediately," tweeted newly-appointed Richard Grenell.

Germany is one of Iran's largest trading partners, with German exports hitting some 2.6 billion euros in 2016, up 26 percent on the previous year, according to the Ministry of Finance in Berlin.

Trump in Washington on Tuesday said he had consulted America's friends in the Middle East and concluded "that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement."

Grenell, a strong backer of the US leader, assumed his role in Berlin on Tuesday just hours before Trump made the announcement which came despite appeals to save the deal from fellow signatories Germany, Britain and France.

Before starting his new role, Grenell previously complained that Germany "should have joined" US, British and French missile strikes against Syria last month over its suspected chemical weapons use.