Iran yesterday confirmed that Russia had agreed to deliver the S-300 air defence system, a move that is likely to irk the Bush administration and gives further proof of Russia and Iran's deepening strategic partnership.

Iran's defence minister, Mostafa Mohammad Najjar, told Iranian TV that the deal had been agreed under a previous "contract". He did not say when the system would be shipped to Iran.

Russian defence experts yesterday acknowledged that the missile system, originally designed in the 1970s, would significantly enhance Iran's ability to shoot down enemy aircraft.

The S-300 had a range far superior to that of the US Patriot system, experts said. It could also shoot down cruise and ballistic missiles, they added.

"It's a formidable system. It really gives a new dimension to Iran's anti-aircraft defences," said one Russian defence expert, who declined to be named.

"It's purely a defensive system. But it's very effective. It's much better than the US system. It has good radar. It can shoot down low-flying cruise missiles, though with some difficulty."

The sale follows Russian president Vladimir Putin's visit to Iran in October to attend a meeting of Caspian Sea nations, the first trip by a Russian head of state to Tehran since Stalin attended a 1943 summit with Churchill and Roosevelt.

Putin has made clear his opposition to the US hardline stance on Iran, and has defended Iran's right to pursue a peaceful nuclear programme.

He has also repeatedly derided the Bush administration's plans to build a missile defence system in Europe to counter a potential nuclear missile fired by Iran or North Korea.

Yesterday Russia's foreign minister, Sergei Lavrov, repeated the Kremlin's contention that the Pentagon's defence shield was aimed at "deterring" Russia, and not at a "hypothetical threat" from Iran.

Lavrov also alleged that the US administration was working to overthrow the Iranian regime. "I think this is a concurrent goal," he said. "If our American partners pursue the aim of regime change... this would be an alteration of policies and we would oppose it."

Earlier this month, Russia delivered the first 80-tonne shipment of fuel to Iran's Bushehr power station, which Russian engineers are controversially building under a $1bn (£504m) contract.

Russia has been Iran's main weapons supplier for several decades. Earlier this year Russia delivered 29 Tor-M1 defence missile systems to Iran under a $700m contract signed in December 2005.

Russian officials would not comment on the Iranian statement, but defence sources told the Interfax news agency that a contract for the missiles' delivery had been signed several years ago. They envisaged the delivery of several dozen S-300 missile systems, they added.

The S-300 is much more powerful and versatile than the Tor-M1. The S-300 is capable of shooting down aircraft, cruise missiles and ballistic missile warheads at ranges of nearly 100 miles, and at altitudes of up to 90,000 feet.

Rumours about the sale of S-300 missile systems to Iran have circulated for a long time, but Russian officials had consistently denied it.

On Tuesday, Russia's military successfully test-fired a new intercontinental ballistic missile capable of carrying multiple nuclear warheads - a weapon intended to replace ageing Soviet-era missiles, Associated Press reported.

The RS-24 missile was launched from the Plesetsk facility in northern Russia, and its test warheads hit targets on the Kura testing range on the Kamchatka peninsula, some 4,340 miles east, strategic missile forces spokesman Alexander Vovk told AP.