United States intelligence sources have determined that Iran could shower Europe with scores of missiles.

Defence Secretary Robert Gates has said his intelligence units had researched Iran’s ballistic missile capability and had decided Europe would have to deal with "scores or even hundreds of missiles" if Iran decided to attack.

Gates said the intelligence had led to the Obama administration's decision to overhaul missile defences in Europe, in and around its NATO allied countries.

He said the upgraded missile interceptors "would give us the ability to protect our troops, our bases, our facilities and our allies in Europe."

Speaking at a congressional hearing, Gates said that Russia's approach to Iran had been "schizophrenic," pursuing commercial ties while acknowledging that a nuclear-armed Tehran would pose a major security threat.

Gates said he had nevertheless sought to allay Russian concerns about the new US approach to European missile defences by playing down the system's ability to counter a large-scale attack from Russia.

He said: "Our missile defences do not have the capability to defend against the Russian Federation's large, advanced arsenal. Consequently, US missile defences do not and will not affect Russia's strategic deterrent."

US intelligence agencies were more concerned, he said, about Iran's growing nuclear missile threat, which is why NATO allies intended covering Europe with anti-ballistic missile systems.