A helicopter carrying Iranian President Ebrahim Raisi and his foreign minister crashed on May 19 as it flew over a mountainous area in Iran through heavy fog.
The fate of Raisi and others onboard is not yet known but an Iranian official told Reuters their lives were "at risk."
"We are still hopeful but information coming from the crash site is very concerning," they added.
Iran is one of Russia's closest allies on the international stage. The two countries have only deepened their military and political cooperation since the start of the full-scale invasion of Ukraine in February 2022.
Most notably, Iran has provided Russia with thousands of Shahed kamikaze drones used in airstrikes against Ukraine and has helped Moscow build a drone factory in Russia, as well as reportedly sending thousands of ballistic missiles.
Iranian state media said the helicopter made a "hard landing" near the city of Tabriz in the north-west of the country.
Rasi and his entourage were returning in a convoy of three helicopters from a trip to the border with Azerbaijan where he opened two new dams.
A rescue operation is underway but is being hampered by both the terrain and weather.
Read also: Explainer: Iran’s cheap, effective Shahed drones and how Russia uses them in Ukraine
