DUBAI (Reuters) - Britain’s ambassador to Tehran has returned to Iran, he announced in an online posting on Thursday, after his departure last month following a brief arrest by Iranian security forces fuelled speculation that he may not be coming back.

“I recently returned from London to Tehran. This was an important trip that was planned some time ago,” Rob Macaire said in Farsi in an Instagram video, noting he met with Britain's foreign minister and other top officials during his trip.

Macaire was briefly detained last month, which Iranian officials said was because he attended an illegal demonstration that took place amid public anger over Iran’s belated admission that its military shot down a Ukrainian airliner, killing all 176 aboard.

The ambassador said he had attended a vigil for victims of the crash and London said his detention was a violation of diplomatic conventions.

Iran’s foreign ministry summoned Macaire, who has been in the post since 2018, to complain over the incident and the judiciary labelled him an “undesirable element”, state media reported.

A prominent hardline Iranian cleric said at the time that expelling the ambassador would be the best option otherwise loyal supporters of a general killed in a U.S. drone strike last month would “chop him to small pieces”.

Tensions in the Middle East escalated last month after the United States killed Iranian military commander Qassem Soleimani in a drone strike, sparking missile attacks by Iran on U.S. bases. Iran said a missile accidently hit the Ukranian jet.

Friction between Tehran and Washington has been building since 2018 when President Donald Trump withdrew the United States from a six-nation nuclear deal in which Tehran agreed to curb its nuclear activities in exchange for the lifting of most sanctions.

Iran announced in January that it would abandon limitations on enriching uranium, taking a further step back from the 2015 nuclear deal, but it would continue to cooperate with the U.N. nuclear watchdog. It says it has never pursued the development of nuclear weapons.

Macaire said in the video that Britain wants to use the dispute resolution mechanism in the multilateral nuclear deal with Iran to find a path forward and that Brexit will not affect its commitment to the deal.

Last month, Britain, France and Germany triggered the deal’s dispute mechanism, which amounts to formally accusing Iran of violating the terms of the agreement and could lead to the reimposing of U.N. sanctions lifted under the deal.

Iran said last month it could quit the global nuclear Non-Proliferation Treaty (NPT) if European countries refer it to the U.N. Security Council over the nuclear agreement.

The 1968 NPT has been the foundation of global nuclear arms control since the Cold War.