US President Donald Trump's new ambassador to Germany, Richard Grenell, arrived in Berlin early Tuesday and presented his credentials after a nearly 16-month gap since his predecessor John Emerson left the country.

Grenell, a vocal critic of the 2015 nuclear agreement with Iran that Germany helped to negotiate, arrived on the day that Trump is expected to pull out of the deal and reinstate sanctions against Tehran.

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 US spokesman at the United Nations from 2001 to 2008, and then worked as a contributor to Fox News.

The US 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 US official.