Yemen's conflict with the rebels has intensified since August, when the army launched Operation Scorched Earth to try to crush an intermittent revolt that first erupted in 2004.
The rebels, known as the Houthis after the family name of their leader Abdul-Malik al-Houthi, are fighting government troops in the north, complaining of social, religious and economic discrimination.
Here is some background about the Houthi rebels:
WHO ARE THE HOUTHIS?
* The Houthis, like most tribesmen in Yemen's northern highlands, belong to the Zaydi sect of Shi'ite Islam, whose Hashemite line ruled for 1,000 years before the 1962 revolution.
* Zaydis, who make up about a third 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 Saudi-influenced 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.
* President Ali Abdullah Saleh, himself a Zaydi, at first used the Houthis to counter-balance the Salafi groups. 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 U.S. cities, Saleh declared support for Washington's "war on terror", in part to enlist U.S. support against the Houthis, whom Yemeni officials accuse of having links to al Qaeda, Iran or Lebanon's Hezbollah.
* 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.
* 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 a year later.
A SAUDI OFFENSIVE:
* In November, Saudi Arabia launched a military offensive against the rebels after a rebel cross-border incursion. Houthi denied charges that infiltrators entered Saudi territory and called the offensive against the group "unjustified and unnecessary" and accused it of mainly targeting civilians through air raids.
* Saudi Arabia claimed victory on Dec. 26 in the conflict, saying the army has driven away the last infiltrators from its territory, an Arabic language daily said.
Sources: Reuters/International Crisis Group
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