TEHRAN - Iran began what it said was its biggest ever air defense drill on Tuesday to test its ability to deter air strikes, which the United States and Israel have not ruled out to prevent it from developing nuclear weapons.

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The website of Iran's English-language Press TV said the five-day war games were being held near nuclear facilities and included tests of long-range missiles.

On Sunday, a senior commander of the elite Revolutionary Guards said land forces had carried out military drills near Iran's nuclear facilities "exactly like real combat".

Western countries suspect Iran's atomic work is a cover for a nuclear weapons programme. Tehran denies this, saying it needs nuclear technology to generate electricity.

Israel, which says a nuclear-armed Iran would be a threat to its existence, and its ally the United States have refused to rule out pre-emptive strikes against Iran, but US Defense Secretary Robert Gates argued strongly against the military option on Tuesday.

"The large-scale military manoeuvres ... will improve readiness to confront possible threats to Iran's air space and the very populated, vital and nuclear centres," Ahmad Mighani, head of an air force unit that responds to threats to Iran's air space, was quoted by state TV as saying.  

Iran has repeatedly announced advances in its military capability to show it is ready to respond to military aggression.

Some Western officials suspect Iran is developing more sophisticated missiles and carrying out well-publicised missile tests so that it can deliver a nuclear weapon.

The Islamic Republic denies this, saying its missile development efforts are for defensive purposes only.

Iran said earlier this month that it had developed a home-made version of the Russian S-300 missile system and would soon test-fire it.  

Moscow supported a fresh round of UN sanctions against Iran in June, and then refused to deliver an order of S-300s to Iran after persistent lobbying by Israel and the United States.

The S-300 is a mobile, long-range air defense system that can detect, track and destroy ballistic missiles, cruise missiles and low-flying aircraft.

Tehran has been hit by various international sanctions for refusing to halt its uranium enrichment activities. Uranium can be enriched to produce fuel for nuclear power plants or, if enriched to a higher level, to make atomic bombs.