The complex of rooms and offices beneath the Abbas Abad district in the north of the capital is designed to serve as a bolthole and headquarters for the country's rulers as military tensions mount.

The recently completed command center is connected by tunnels to other government compounds near the Mossala prayer-ground, one of the city's most important religious sites.

Offices of the state security forces, the energy department and the Organization of Islamic Culture and Communications are all located in the same area. The construction of the complex is part of the regime's plan to move more of its operations beneath ground.

The Revolutionary Guard has overseen the development of subterranean chambers and tunnels - some more than a kilometer long and an estimated 11 meters high and wide - at sites for research and development work on nuclear and rocket program.

The opposition National Council of Resistance of Iran learned about the complex from its contacts within the regime. The same network broke the revelation in 2002 that Iran had been operating a secret nuclear program for 18 years.

The underground strategy is partly designed to hide activities from satellite view and international inspections but also reflects a growing belief in Teheran that its showdown with the international community could end in air strikes by America or Israel.

"Iran's leaders are clearly preparing for a confrontation by going underground," said Alireza Jafarzadeh, the NCRI official who made the 2002 announcement. America and Europe believe that Iran is secretly trying to acquire an atomic bomb, although the regime insists that its nuclear program is for civilian energy purposes.

As the United Nations Security Council prepares to discuss Iran's nuclear operations this week, the country has been stepping up plans for confrontation. Its chief delegate on nuclear talks last week threatened Teheran would inflict "harm and pain" on America if censured by the Security Council.

Mahmoud Ahmadinejad, the hardline president who has called for Israel to be "wiped off the map," also warned that the West would "suffer" if it tried to thwart Iran's nuclear ambitions.

As the war of words intensified, President George WBush said Teheran represents a "grave national security concern" for America.

In Iraq, which Ahmadinejad hopes will develop into a fellow Shia Islamic state, Iran is already using its proxy militia to attack British and US forces, often with Iranian-made bombs and weapons.

As tensions grow, Teheran could order Hezbollah - the Lebanese-based terror faction that it created and arms - to attack targets in Israel.

The regime is also reviewing its contingency plans to attack tankers and US naval forces in the Persian Gulf and to mine the Strait of Hormuz, through which about 15 million barrels of oil (about 20 percent of world production) passes each day. Any action in the Gulf would send oil prices soaring - a weapon that Iran has often threatened to wield.

The Pentagon's strategic planning is focused on the danger that Iran might try to mine the strait and deploy explosive-packed suicide boats against its warships.

In May, US vessels in the Gulf will take part in the Arabian Gauntlet training exercise that deals with clearing mines from the strait, which has a navigable channel just 3.2km wide.

The naval wing of the Revolutionary Guard has in recent years practised "swarming" raids, using its flotilla of small rapid-attack boats to simulate assaults on commercial vessels and US warships, according to Ken Timmerman, a US expert on Iran.

US intelligence believes that if Iranian nuclear facilities were attacked by either America or Israel, then Teheran would respond by trying to close the Strait of Hormuz with recently upgraded naval forces, mines and anti-ship cruise missiles.

THE SUNDAY TELEGRAPH