[FACTBOX] Roots of Yemen's conflict with northern rebels

Here is some background about the rebels, known as Houthis after the family of their present leader, Abdul-Malek al-Houthi:

Zaydis, a substantial minority in Yemen, have coexisted easily with majority Sunnis in the past

* The Houthis, like most tribesmen in Yemen's northern highlands, belong to the Zaydi sect of Sh'ite Islam, whose Hashemite line ruled for 1,000 years before the 1962 revolution.

* Zaydis, a substantial minority of Yemen's 23 million people, have coexisted easily with majority Sunnis in the past, but Badr al-Din al-Houthi, a cleric from the northern province of Saada, promoted Zaydi revivalism in the 1970s, playing on fears that Salafis threatened Zaydi identity.

* After north and south Yemen united in 1990, the movement spawned the al-Haq party and the Houthi-led Believing Youth group. Houthi's son Hussein was elected to parliament in 1993. Saada remained neglected economically by the Sanaa government.

Conflict began after Houthis embarrassed Saleh

* President Ali Abdullah Saleh, himself a Zaydi, at first used the Houthis to counter-balance the Salafi outfits. The government later portrayed Believing Youth as a fundamentalist group out to subvert the state and restore the Zaydi imamate.

* After the Sept. 11, 2001 attacks on the United States, Saleh declared support for Washington's "war on terror", in part to enlist U.S. support against the Houthis, accused by some Yemeni officials of having links to al Qaeda, Iran or Hezbollah.

Security forces killed Hussein al-Houthi in September 2004

* The Houthis retort that the government, with U.S. and Saudi backing, is targeting Zaydis in general, forcing them to take up arms to defend their villages against "oppression".

* Conflict began after Houthis embarrassed Saleh by shouting "Death to America, death to Israel, a curse on the Jews, victory to Islam" in his presence in a Saada mosque in 2003.

Hussein al-Houthi

* Security forces killed Hussein al-Houthi in September 2004, only for further rounds of fighting to erupt in the mountains around Saada city, each more violent than the last.

* Qatar brokered a short-lived ceasefire in June 2007 and sponsored a peace deal signed in February 2008, but clashes soon broke out again. Saleh unilaterally declared the war over in July last year. Full-scale fighting resumed late last month.