DUBAI, May 19 (Reuters) - A helicopter carrying Iranian President Ebrahim Raisi and his foreign minister crashed on Sunday as it was crossing mountain terrain in heavy fog, an Iranian official told Reuters, and rescuers were struggling to reach the site of the incident.

The official said the lives of Raisi and Foreign Minister Hossein Amirabdollahian were "at risk following the helicopter crash", which happened on the way back from a visit to the border with Azerbaijan in Iran's northwest.

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

Iranian state media said bad weather was the cause of the crash and was complicating rescue efforts. The chief of staff of Iran's army ordered all the resources of the army and the elite Revolutionary Guard to be put to use in search and rescue operations.

"It is dark and it has started raining, but the search continues. Rescue teams have reached the area ... however, the rain has created mud, making the search difficult," a local reporter told state TV.

State TV had earlier stopped all its regular programming to show prayers being held for Raisi across the country and, in a corner of the screen, live coverage of rescue teams deployed on foot in the mountainous area in heavy fog.

The rescue teams were expected to reach the probable site of the crash later on Sunday evening.