Suicide bomber in Yemen kills 20 Houthi fighters: sources

There was no immediate claim of responsibility nor confirmation of the attack from the government, whose forces barely operate in the area, but the leader of al Qaeda's local branch has vowed to respond to the Houthi capture of his group's strongholds in al-Manasseh area in the Qifa tribal region.

"We will turn Qifa lands into an area of mass graves for them (the Houthis)," Nabil al-Dahab, whose insurgent group operates in the town of Radda, said in a statement on Monday.

Violence has spread and intensified in Yemen since the Houthis captured the capital Sanaa on Sept. 21.

Once an obscure religious movement in Yemen's north seeking greater autonomy, they have established themselves as power-brokers and have sent their militiamen into the west and center of the country, far beyond their traditional redoubts.

Al-Bayda province, the scene of Sunday's attack, is home to heavily armed and restive Sunni tribes who fought with the central government even in more stable times.

"The suicide bomber drove a car, attacking a group of Houthis in the al-Manassah district and killed 20 of them," a tribal leader told Reuters on condition of anonymity,

Scores have been killed in the province in the last week as tribesmen, along with fighters from al Qaeda's affiliate in Yemen, have clashed with the Houthis.

The al Qaeda branch has drawn many fighters from local tribes who oppose the recent Houthi dominance in central Yemen, an al Qaeda stronghold. The insurgents denounce the Yemeni government as a pawn of the West and the Houthis as apostates.

The United States and its Gulf allies fear the spread of sectarian violence in Yemen - situated between Saudi Arabia and an important shipping route on the Red Sea - will unravel the government's already limited control over its territory.