Yemeni security and tribal sources say Shi'ite rebels have attacked a Sunni Islamist school and other sites in the northern province of Saada, killing at least 21 people and wounding about 50 others.

A provincial security official and local Sunnis told foreign news agencies that the Shi'ite Zaidi rebels known as Houthis shelled Sunni targets in the town of Damaj late Saturday and Sunday. They say the targets included a school run by the Sunni Islamist Salafi sect.

The Shi'ite Zaidi sect dominates the Saada region and sees the Salafis as a threat. Militants from both sides have fought each other in recent weeks, prompting local tribesmen to try to secure a truce.

Houthi rebels led an uprising against the Yemeni government in the country's north for years before both sides agreed to a truce last year.

In another development, Yemeni President Ali Abdullah Saleh returned to the capital, Sana'a, late Saturday after a three-day visit to Saudi Arabia, where he signed a Gulf-sponsored agreement to transfer power to his deputy as part of a transition to democracy.

Earlier in the day, Yemeni Vice President Abd-Rabbu Mansour Hadi declared February 21, 2012 as the date for a presidential election to formally end Mr. Saleh's 33 years in power. The Gulf plan agreed to by Mr. Saleh and Yemen's main political parties allows him to remain president in an honorary capacity until that election.

Mr. Saleh has faced 10 months of nationwide mass protests against his autocratic rule. Many of the protesters are upset that the Gulf Cooperation Council plan gives him immunity from prosecution when he leaves office.