Despite demands by global and regional powers that he step down, Saleh has refused to sign a deal aimed at transferring power and averting a civil war that could destabilize the world's main oil-exporting region.

Global powers are worried that Yemen could turn into a failed state, fears that have been heightened since Al Qaeda and other Islamists seized Zinjibar a few days ago.

Three Yemeni soldiers were killed in an ambush near Zinjibar on Monday, a security official said, a day after a similar incident claimed the lives of six troops. Residents said fighter jets strafed militant areas around Zinjibar but also hit buildings in the town of 20,000, killing at least 13 people.

Medical workers in Taiz said soldiers had opened fire on a demonstration late on Sunday and hit protesters with bulldozers, killing at least 15 and wounding hundreds in what the United States described as an "unprovoked and unjustified attack."

Al Jazeera said in an unsourced report that 57 people had been killed in Taiz over the past two days.

In the capital Sanaa, several explosions and shooting were heard late on Monday in the district of Hasaba, the scene of week-long fighting between Saleh's forces and a rival tribe.