A helicopter carrying Iranian President Ebrahim Raisi and his foreign minister has crashed as it was crossing mountain terrain in heavy fog on the way back from a visit to the country's border with Azerbaijan, an Iranian official told Reuters.

Iranian news agency IRNA said the helicopter involved in the incident had been carrying Mr Raisi as well as Foreign Minister Hossein Amirabdollahian and local officials.

The official said the lives of Mr Raisi and Foreign Minister Hossein Amirabdollahian were "at risk following the helicopter crash".

"We are still hopeful but information coming from the crash site is very concerning," said the official, speaking on condition of anonymity.

The semi-official Fars news agency called on Iranians to pray for the president following the reports.

Interior Minister Ahmed Vahidi told state TV only that one of the helicopters in a group of three had come down hard, and that authorities were awaiting further details.

Rescuers were attempting to reach the site, state TV said, but had been hampered by poor weather conditions. There had been heavy rain and fog reported with some wind.

IRNA called the area a "forest" and the region is known to be mountainous as well.

Iran flies a variety of helicopters in the country, but international sanctions make it difficult to obtain parts for them. Its military air fleet also largely dates back to before the 1979 Islamic Revolution.

Mr Raisi, 63, was elected president at the second attempt in 2021, and since taking office has ordered a tightening of morality laws, overseen a bloody crackdown on anti-government protests and pushed hard in nuclear talks with world powers.

In Iran's dual political system, split between the clerical establishment and the government, it is the supreme leader rather than the president who has the final say on all major policies.

But many see Mr Raisi as a strong contender to succeed his mentor, Supreme Leader Ayatollah Ali Khamenei, who has strongly endorsed Mr Raisi's main policies.

Reuters/AP