The United States Ambassador to Germany warned Iranian Foreign Minister Mohammad-Javad Zarif on Wednesday about a threat made against the British ambassador in Tehran.

The United States envoy, Richard Grenell, in a tweet reacted strongly to an Iranian cleric's threats against the British ambassador and said this should be "loudly and widely condemned in the United States, throughout Europe and everywhere. This can't be ignored" and addressed Zarif directly saying: "Watch!"

The British ambassador's Rob Macaire was detained by Iranian security forces on January 12 after attending a vigil for the victims of the Ukrainian plane that was downed by the Revolutionary Guard last week. They subsequently released him after the Iranian foreign ministry intervened.

Following the incident, Seyed Ahmad Alam ol-Hoda, a hardliner cleric and the Supreme Leader's representative in Khorasan Province, called for the expulsion of the British ambassador and said he "should be chopped into pieces".

Iranian hardliners have held several rallies in front of the British embassy in Tehran since Macaire's release and have set his pictures on fire. Ayatollah Ahmad Jannati, the hardliner Chairman of the Guardian Council has also called on the Foreign Ministry to expel the British envoy. "The Foreign Ministry should expel him from the country as persona non-grata," Jannati said on Thursday.

Macaire was summoned to the Iranian Foreign Ministry on January 13 and on the same day Britain's Foreign and Commonwealth Office (FCO) summoned Iran's envoy Hamid Baeidinejad for explanations.

Tensions are high between Iran and the United Kingdom, particularly after Britain, Germany and France which form the European side of the nuclear agreement with Iran (known as JCPOA) triggered the dispute mechanism of the agreement which can potentially reinstate UN sanctions on Iran.

On January 15 the British ambassador returned to London and FCO sources told the press he was back for a planned visit and would return to Tehran.