Iran removed the core of the Arak facility and filled part of it with cement as part of a 2015 deal that gave the country relief from sanctions in return for curbs on its nuclear programme.

Located southwest of Tehran, the reactor is to be modernised with the help of foreign experts under the deal known as the Joint Comprehensive Plan of Action.

"A team of British nuclear experts led by UK Chief Scientific Adviser Professor Robin Grimes arrived in Tehran today to take forward the next stages of the modernisation of the Arak reactor, alongside a team of Chinese experts," said the British embassy.

"The experts will hold talks with the Atomic Energy Organisation of Iran on international technical assistance to the reactor construction," it said in a statement.

The British experts would remain in Iran for three days, the embassy told AFP.

"This visit forms part of our commitment to ensuring that the Joint Comprehensive Plan of Action (JCPOA) provides benefits for both Iran and the wider international community," said the statement.

"That is why we are upholding our obligations to cooperate with Iran to modernise the Arak reactor, helping Iran to develop a modern and up to date civil nuclear programme.

"Our work with Iran on the Arak project has made important progress in the past year," it said.

Tensions have been escalating between Iran and the United States since May last year when President Donald Trump pulled out of the nuclear accord and began reimposing sanctions.

The remaining partners in the deal with Iran include Britain, China, France, Germany and Russia.

The European parties have repeatedly said they are committed to saving the accord, but their efforts have so far borne little fruit.

Tehran has already hit back three times with countermeasures in response to the US withdrawal from the nuclear deal.

On July 1, Iran said it had increased its stockpile of enriched uranium to beyond a 300-kilogramme maximum set by the deal, and a week later, it announced it had exceeded a 3.67-percent cap on the purity of its uranium stocks.

In its latest move it fired up advanced centrifuges to boost its enriched uranium stockpiles on September 7.