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.