SANAA, Oct 10 (Reuters) - Yemen security forces said they have killed 100 rebels and injured a further 280 in the country's northern Saada province, where the authorities are battling a renewed Shi'ite revolt. The conflict has intensified since the army began "Operation Scorched Earth" on Aug. 11, and aid agencies are warning of a humanitarian crisis in the north, where up to 150,000 people have fled their homes since Shi'ite tribesmen launched an insurgency in 2004.

"Terrorist and destructive elements yesterday evening infiltrated (areas) between military barracks and security posts in Saada province," the military said in a statement issued late on Friday.

"Our armed and security forces put a stop to them and inflicted painful and heavy blows on them during which the terrorist 'Houthi' elements lost more than 100 people and more than 280 were injured," the statement said, referring to rebels led by Abdul-Malik al-Houthi.

Washington and Riyadh fear that fighting in northern Yemen, and frequent street clashes with separatists in the south, may create instability that al Qaeda could exploit to carry out attacks in Saudi Arabia. It has already staged a comeback in Yemen in the past two years, with attacks on government and foreign targets.

The northern Zaydi rebels say they suffer religious discrimination by Sunni fundamentalists who have gained in strength because of President Ali Abdullah Saleh's close ties to Saudi Arabia, which adheres to a puritanical form of Sunni Islam.

Yemen, the Arab world's poorest country, is also facing a growing al Qaeda militancy.

Yemeni authorities have barred journalists and diplomats from travelling independently to the rugged provinces of Saada, Amran and Hajjeh, where troops and pro-government tribesmen are battling the well-armed Houthis and their tribal allies. (Reporting by Mohammed Sudam; writing by Raissa Kasolowsky)

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