The reported sale of advanced anti-aircraft missiles to Tehran has made the need to deal with Iran’s nuclear programme more urgent than ever.

A senior source in Russia’s Federal Bureau of Military-Technological Cooperation confirmed to the Interfax news agency on Wednesday that his country had signed a contract to sell the S-300PMU missile system to Iran in 2007. The official said that the system had not yet been supplied.

Israel has been lobbying Russia for the last two years not to supply the equipment — believed to be one of the most advanced anti-aircraft missile systems in the world — to the Iranians. The S-300PMU (known to Nato as the SA-20 “Gargoyle”) was developed as a dual anti-aircraft and anti-ballistic missile weapon. It can hit targets as far away as 150 km and its radar is capable of tracking up to 100 objects simultaneously. Prime Minister Ehud Olmert travelled to Moscow last October specifically to ask the Kremlin not to supply the missiles to Iran.

Israeli military experts fear that the S-300 would confer near-immunity from aerial attack and at the least inflict heavy losses on any air force, American or Israeli, attempting to launch a widespread strike on the country’s nuclear installations. The United States Air Force has already started to acquire advanced F-22 and F-35 fighter jets with “stealth” capability, enabling it to evade the S-300 radar, but Israel will be able to receive similar planes only from 2015, a date by which the Iranians are almost certain to have built nuclear bombs.

Despite previous reports that Iran has already acquired S-300s, the Russians insist they have not yet supplied the system. If a supply date is indeed imminent, it could drastically narrow the window of opportunity that Israel and the US have to act militarily against the Iranian nuclear programme.