Factbox - Leading factions in Yemen's crisis

(Reuters) - The crisis in Yemen cuts through the country's political, tribal, regional and sectarian layers to create a complex conflict that risks sucking in neighbouring oil giant Saudi Arabia and its main regional rival, Iran. These are some of the most important factions. * THE HOUTHIS, OR ANSARULLAH The group began as a movement of young men called the Believing Youth set up in 1992 to back the rights of the Zaydi Shi'ite sect that makes up around a fifth of Yemenis. It fought the state from 2003-09 and more recently claimed the mantle of a national revolution, sweeping south and seizing Sanaa. The Houthis are allied to Iran, which has trained, armed and funded the group, Yemeni and Iranian officials have said. The Houthis deny receiving Iranian military training. * ABD-RABBU MANSOUR HADI Elected as an interim president in 2012 to lead a transition towards democracy, President Abd-Rabbu Mansour Hadi was besieged in his residence by the Houthis after they seized Sanaa. The former army general resigned in January and was put under house arrest. Hadi escaped last month to Aden, repudiated his resignation and formed a government there. He called on the army to join him. * ALI ABDULLAH SALEH The ruler of north Yemen from 1978 and of the unified state from 1990 was forced to concede power in 2011 after mass protests, although he remained ceremonial president until 2012. Western countries accuse him of using his wide influence, his military power base, and an unlikely alliance with the Houthis to undermine Hadi in an attempt to win back power. * AL QAEDA IN THE ARABIAN PENINSULA AQAP has been the most active wing of the Islamist militant movement for years, plotting attacks on international airliners and launching raids into Saudi Arabia. Despite repeated army campaigns to oust it from its strongholds in the south and east, it has carried out a deadly raids against Yemen's security forces and claimed responsibility for an attack on the Charlie Hebdo magazine in Paris that killed 12 in January. * SOUTHERN HIRAK The movement is an unwieldy coalition of groups who want to reverse the state's 1990 reunification and revive the old South Yemen. Hirak can mobilise large numbers on the streets of southern cities such as Aden, but has no coherent leadership to translate its popular support into action. * ISLAH A party which combines Islamist and tribal interests, Islah has widespread support across Yemen and looked poised to win more power during the transition, but it lost out in the Houthi advance. Its military might came from an alliance with General Ali Mohsen al-Ahmar, who held the loyalty of key brigades, but has fled to Saudi Arabia. (Compiled By Angus McDowall; editing by Noah Browning, William Maclean, Sami Aboudi, Louise Heavens and Giles Elgood)