Russia previously supplied its S-300 anti-aircraft and missile defence system to Tehran, but western threats of sanctions and a UN embargo on supplying Iran with weapons, or purchasing Iranian systems, held Moscow back.
Russia completed S-300 deliveries to Iran in 2016, but Tehran has long wanted the Sukhoi Su-35 fighter jet, a modern variant of the 1980s-era Su-27 fighter, vaunted for its manoeuvrability.
Iran has also supplied Russia with thousands of Shahed-136 one-way attack drones, or “kamikaze” drones. The cheap and expendable explosive aircraft have done a great deal of damage to Ukraine’s energy infrastructure. Washington fears Iran will send ballistic missiles to boost Moscow’s arsenal.
The supply of drones has provoked anger in western countries that have sent about $200 billion in military and economic aid to Ukraine, including air defences, since the Russia began its invasion of Ukraine in February 2022.
"Plans have been finalised for Sukhoi Su-35 fighter jets, Mil Mi-28 attack helicopters and Yak-130 jet trainers to join the combat units of Iran's army," the state-linked Tasnim news outlet quoted Iran's deputy defence minister Mehdi Farahi as saying.
The Su-35, reportedly 24 of them, would be a formidable addition to Iran’s air force, which still flies aircraft from the 1970s, including US-made F-14 Tomcats and F-4 Phantoms. The latter dates back to the Vietnam War.
It is not clear how much threat the Russian jets could pose to Iran’s rivals in the region, including Israel.
The Russian “air supremacy” fighter jet is said to have a smaller “radar cross section", or radar visibility than the US and Israeli-operated F-16. Both types of jet are said to appear on radar as objects several metres across.
But the Su-35’s radar and other sensors would struggle to detect stealth aircraft including the F-35, which is also operated by Israel, and is said to have a radar cross section measured in centimetres, making it almost undetectable except at close range.