The air force of sanctions-hit Iran has an ageing fleet of aircraft and has struggled to acquire spare parts to keep its warplanes in the air.

In a statement to the United Nations, Tehran said it began approaching "countries to buy fighter jets" to replenish its fleet in the wake of the 1980-88 Iran-Iraq war.

"Russia announced it was ready to sell them" after the expiry in October 2020 of restrictions on Iran purchasing conventional weapons under UN Resolution 2231, said the statement carried late Friday by the official IRNA news agency.

"The Sukhoi 35 fighter jets were technically acceptable for Iran," it added.

Tehran has forged strong ties with Moscow in various sectors including the military in the past year.

Kyiv has accused Tehran of supplying Moscow with Shahed-136 "kamikaze" drones used in attacks on civilian targets since Russia's invasion of Ukraine in February last year -- an allegation the Islamic republic denies.

The United States has expressed alarm over the growing military cooperation between Iran and Russia, with Pentagon spokesman John Kirby warning in December that Russia looked likely to sell Iran its fighter jets.

Kirby maintained that Iranian pilots had reportedly been learning to fly the Sukhoi warplanes in Russia, and that Tehran may receive the aircraft within the next year, which would "significantly strengthen Iran's air force relative to its regional neighbours".

Iran currently has mostly Russian MiG and Sukhoi fighter jets that date back to the Soviet era, as well as some Chinese aircraft, including the F-7.

Some American F-4 and F-5 fighter jets dating back to before the 1979 Islamic Revolution are also part of its fleet.

The United States began reimposing sanctions on Iran in 2019, a year after its unilateral withdrawal from the Iran nuclear deal under then-president Donald Trump.

The 2015 deal formally known as the Joint Comprehensive Plan of Action, or JCPOA, gave Iran relief from international sanctions in return for curbs on its suspect nuclear programme.