FACTBOX: The Revolutionary Guards: custodians of Iran's rulers

(Reuters) - An Iranian military commander has said the Basij militia, commanded by Iran's Islamic Revolutionary Guards Corp (IRGC), could disrupt vital Gulf oil shipping routes with its "martyrdom-seeking" volunteers, if the need arose.

The United States has branded the IRGC a proliferator of weapons of mass destruction, said one unit backed terrorists and slapped sanctions on firms and individuals linked to the force. Iran dismissed the accusations and brushed off sanctions.

Following are some questions and answers about the IRGC:

WHAT IS THE ISLAMIC REVOLUTIONARY GUARDS CORPS?

The IRGC was set up after the 1979 Islamic revolution to protect the clerical ruling system and revolutionary values. It answers to Supreme Leader Ayatollah Ali Khamenei, Iran's top authority.

The IRGC has an estimated 125,000-strong military with army, navy and air units. It commands the Basij religious militia, a volunteer paramilitary force loyal to the revolution. Basijis mounted "human wave" attacks against Iraqi troops. In peacetime, they enforce Iran's Islamic social codes. Analysts say Basiji volunteers may number in millions, with 1 million active members.

The Qods (Jerusalem) force is also under IRGC command. It handles the IRGC's foreign activities. The United States says it backs militants in Iraq and Lebanon's Hezbollah guerrillas.

WHAT HAS THE IRGC SAID ABOUT THE NUCLEAR ROW WITH THE WEST?

The Guards have warned they could disrupt oil traffic in the Gulf waterway if pushed but commanders doubt the United States will strike because it is bogged down in Iraq and Afghanistan, and the proximity of U.S. forces to Iran make them too vulnerable to an Iranian response.

WHAT ARE THE IRGC'S MILITARY CAPABILITIES?

Although Guardsmen fought in conventional battles against Iraq, military experts say they developed "asymmetric" tactics, such as hit-and-run raids using small craft targeting shipping in the 1980s when Iran and Iraq sought to knock out each others oil exports.

Such tactics could be used again to disrupt oil shipping lines in the Gulf waterway.

The IRGC has been involved with handling Iran's most advanced missile systems like the Shahab-3 with a range of 2,000 km (1,250 miles), analysts say.

WHAT IS THE IRGC'S POSITION IN THE POLITICAL SYSTEM?

The IRGC's mandate to protect revolutionary values has prompted it to speak out when it felt the system was threatened.

Former IRGC officers have reached top political posts, notably President Mahmoud Ahmadinejad. But views of ex-officers are not homogenous and analysts say inside the IRGC there are different camps, some with a more pragmatic approach.