The whereabouts of Iranian President Ebrahim Raisi and other senior officials remain unknown as international teams join local rescuers in the search in northwestern Iran.

Fog and rain in the province of East Azarbaijan hampered the search in which Turkiye, Azerbaijan, Russia, the EU, among others, are taking part, from Sunday until early Monday, according to Iran's state-run IRIB TV.

Iran's Vice-President for Executive Affairs Mohsen Mansouri, speaking near the possible scene, said the search and rescue work was going on slowly as the area was hard to reach, and bad weather had made the operation more difficult, Xinhua reported, citing local TV reports.

The incident occurred on Sunday in the Varzaqan county, some 670 km away from the capital Teheran.

Millions of Iranians are praying for good news about their president, according to CGTN.

Iran's Foreign Minister Hossein Amir-Abdollahian, East Azarbaijan's Governor Malek Rahmati, and the Friday prayers leader Tabriz Mohammad Ali Ale-Hasehem were also on the helicopter when it suffered from a "hard landing" due to the bad weather, Iranian Interior Minister Ahmad Vahidi told IRIB TV.

The Iranian president was returning home from an event jointly held with Azerbaijan authorities on Sunday.

Raisi was traveling in a convoy of three helicopters when the one carrying him lost communication with the other two about 30 minutes after takeoff.

The two other helicopters immediately began a search of the area and continued the search for about 20 minutes, but were later compelled to make an emergency landing due to thick fog and other weather conditions, Mansouri said.

Xinhua contributed to this report.