SAN'A, Yemen - Two car bombs in central Yemen on Tuesday killed at least 25 people, nearly two-thirds of them schoolgirls whose bus was hit, Yemeni officials said.

A Shiite Muslim rebel group blamed al-Qaeda for causing the girls' deaths in a botched attack on a Shiite official under the rebels' protection.

Impoverished but strategic Yemen, long beset by pervasive tribal and sectarian violence that a weak central government has been unable to rein in, entered a phase of even more serious instability after Shiite Houthi rebels overran the capital, San'a, in September and seized swaths of territory in other parts of the country.

The Houthis, aligned with Shiite Iran, have been locked in combat with the Yemen affiliate of al-Qaeda, which has been a target of repeated U.S. drone strikes. On Dec. 6, al-Qaeda captors executed an American hostage, journalist Luke Somers, together with a South African national being held with him as U.S. special forces backed by Yemeni troops attempted a rescue, the Pentagon said.

In San'a, Houthis were reportedly massing Tuesday near the Defense Ministry, with tensions running high after an incendiary speech a day earlier by a Houthi leader. Over the weekend, the Houthis claimed to have gained control of Arhab, an outlying district of San'a, cementing their grip on the capital.

At least one bomb that went off Tuesday in Bayda province, south of San'a, was believed to have targeted a checkpoint outside the home of a Shiite official, Abdullah Idris. But one of the explosions hit a passing bus carrying female elementary-school students, killing at least 15 of them, according to an unidentified military official quoted on a website associated with the Defense Ministry.

The 10 other dead in Tuesday's blasts in the city of Radaa included Houthi rebels and civilians, but the breakdown was not immediately clear. The area, long an al-Qaeda stronghold, has been a flash point for fighting between al-Qaeda and Houthi forces.

The growing turmoil in Yemen, alongside oil-shipping routes, has alarmed Saudi Arabia, the main regional Sunni power, which accuses Iran of fomenting violence.