BERLIN - The United States Ambassador to Germany Richard Grenell has resigned from post and his deputy has temporarily filled in, local media Deutsche Presse Agentur (DPA) reported on Tuesday.

Grenell confirmed his retreat late Monday on his Instagram account without indicating his next post.

It has been widely speculated that he may join US President Donald Trump's re-election campaign, according to the DPA.

Grenell's deputy Robin Quinville has become the chargee d'affaires until a new ambassador comes.

During his tenure of more than two years in Germany, Grenell was seen widely by German media as an aggressive, unconventional and even rude diplomat, as he zealously presented Trump's policies in Germany and Europe.

Shortly after his appointment in May 2018, he directly warned German enterprises not to work with Iran, which was criticized for meddling in other country's internal affairs.

He tweeted many controversial statements, following Trump's rhetoric, including the accusation about Germany's insufficient military and the threat to withdraw U.S. troops from Germany.

He has also long been finding things to pick at about the Nord Stream 2 project, which is designed to transport natural gas from Russia to Germany through the Baltic Sea.

Although German ministers have refrained from radical responses to Grenell, parliament members have often openly denounced his remarks, and some lawmakers even asked foreign ministry to declare him as "persona non grata" -- an unwelcome or unacceptable person in diplomacy.