BERLIN (Reuters) - U.S. President Donald Trump’s new ambassador to Germany, Richard Grenell, warned German companies to halt activities in Iran after the U.S. leader said Washington was pulling out of the Iran nuclear deal and would reimpose sanctions.

Grenell, a long-time critic of the 2015 nuclear deal, posted the warning on Twitter just hours after arriving in Berlin and presenting his credentials to the German president.

Trump made the announcement in a televised address from the White House, in a move that will raise the risk of conflict in the Middle East, upset Germany and America’s other European allies, and unsettle global oil markets.

“US sanctions will target critical sectors of Iran’s economy,” Grenell said in his first posting on Twitter after arriving in Germany. “German companies doing business in Iran should wind down operations immediately.”

German industry reacted nervously to the U.S. announcement, warning that European companies could face U.S. fines if their Iranian business partners wound up on U.S. sanctions lists.

Grenell is starting his job in Berlin nearly 16 months after his predecessor John Emerson left the country.

A Republican foreign policy writer and avid Twitter user, Grenell was an aide to Trump’s new national security adviser, John Bolton, when Bolton was former Republican President George W. Bush’s ambassador to the United Nations. Grenell served as the U.S. spokesman at the United Nations from 2001 to 2008, and then worked as a contributor to Fox News.

The U.S. Senate confirmed Grenell on April 26, making him the highest-ranking openly gay official in the Trump administration. The next day, he joined Trump at the White House to welcome Chancellor Angela Merkel for talks that produced little movement on trade disputes or the Iran accord.

Trump had nominated Grenell in September but key Democrats help up his confirmation, citing concerns about what they called Grenell’s history of making statements insulting to women politicians on the internet and during television appearances.

Asked about such comments during his confirmation hearing, Grenell said he was trying to be funny, not insulting, and would not have made such statements while serving as a U.S. public official.