At least 20 gunmen were killed Thursday in clashes between Shiite Houthi rebels and Sunni Islamist Salafi fighters in Yemen's northwest restive province of Hajja, a provincial security official said.

"Twenty gunmen, mostly from the Shiite rebels, were killed, while dozens of people from both sides were wounded," the official told Xinhua by phone on condition of anonymity.

"The clashes, triggered by conflicts related to the disputes over religion difference, flared in the morning in several villages bordering the northern province of Saada, a stronghold of the Houthi-led Shiite rebels who have engaged in fighting with outgoing President Ali Abdullah Saleh's government forces over the past six years," the official said.

Since the protests demanding the end of Saleh's 33-year rule erupted in Yemen in late January last year, Houthi rebels have been engaging in battles against northern Sunni tribes in an attempt to expand their control over the northern provinces of Saada, Amran and Hajja.

On Aug. 26, 2010, the Yemeni government and the Shiite group signed an agreement in Doha to cement a fragile cease-fire to end the sporadic battles since 2004, but the rebels' clashes with local tribesmen and Sunni supporters are still rocking the region.