FACTBOX: How big is Iran's military?

(Reuters) - The United States, leading efforts to isolate Iran because it believes Tehran is trying to build atomic bombs, has said it wants diplomacy to end the standoff, but has refused to rule out military options if that fails.

Iran insists its nuclear plans are peaceful but has said its forces are ready to respond to any threat.

Following are details of Iran's military:

* ARMED FORCES:

-- TOTALS: Iran has 545,000 men active. Ataollah Salehi is the Armed Forces Chief.

* ARMY: The army comprises 350,000 men. Of those 220,000 are conscripts. The Islamic Revolutionary Guards Corp, viewed as the most loyal guardians of the ruling system, has another 125,000 men. By 2004 the army was organized in four corps, with four armored divisions and six infantry divisions.

-- Totals of equipment include those held by the Revolutionary Guards.

-- There are nearly 1,700 tanks which comprises around 1,600 battle tanks. There are 100 Zulfiqar locally produced main battle tanks. There are also some 150 M-60s and 75 Russian T-62s as well as 100 British chieftain tanks. There are 640 armored personnel carriers. There are 8,196 artillery pieces of which 2,010 are towed, 310 are self-propelled.

MISSILES:

-- In a 2007 parade to mark the 1980-88 Iran-Iraq war, Iran showed among its weaponry the Shahab-3 missile, saying it could reach 2,000 km (1,250 miles) -- enabling it to hit Israel and U.S. bases in the region. Another missile at the parade, Ghadr-1, can reach targets 1,800 km (1,125 miles). It was believed to be the first time it has been shown publicly.

* NAVY: There are 18,000 naval personnel. The navy has its headquarters at Bandar-e Abbas. Iran's navy has three Russian Kilo class submarines and three frigates and two corvettes. It has around 250 Patrol and Coastal Combatants which includes 10 patrol craft with surface to surface missiles. It also has mini submarines.

* AIR FORCE:

-- The air force has 52,000 personnel. There are 281 combat aircraft. However, service ability may be around 60 percent for U.S. aircraft types and 80 percent for Russian aircraft. This total also includes Revolutionary Guards equipment. There are F-14 and MiG 29 aircraft. There are also some aircraft impounded from Iraq - Sukhoi Su 24s and 25s. Iran also has transport aircraft and helicopters.

-- In September 2007 Iran said it had tested two new generation, domestically-produced jet fighter planes. State tv said the Saegheh jet was a new generation of the Azarakhsh (Lightning) fighter, which Iran said in August was being built on an industrial scale.

Sources: Reuters/Military Balance 2006/www.globalsecurity.org

(Writing by David Cutler; Editing by Samia Nakhoul)