"Raisi was an outstanding politician whose entire life was dedicated to serving his homeland," Putin said in a letter to supreme leader Ayatollah Ali Khamenei, published on the Kremlin's website.

"As a true friend of Russia, he made an invaluable personal contribution to the development of good-neighbourly relations between our countries, and made great efforts to take them to the level of a strategic partnership," he added.

The Kremlin leader said he was sending condolences to the leader of Islamic Republic of Iran and the Iranian people "in the face of such a grave and irreplaceable loss."

Raisi and his companions were martyred after helicopter carrying them crashed on Monday.