UPDATE 1-Yemeni protesters storm U.S. embassy in Sanaa-witnesses

DUBAI, Sept 13 (Reuters) - Hundreds of Yemeni demonstrators stormed the U.S. embassy in Sanaa on Thursday in protest at a film they consider blasphemous to Islam, and security guards tried to hold them off by firing into the air.

The attack followed Tuesday night's storming of the United States Consulate in Benghazi, where the ambassador and three other staff were killed. President Barack Obama said the perpetrators would be tracked down and ordered two destroyers to the Libyan coast, but there were fears protests would spread to other countries in the Muslim world.

Witnesses in Sanaa said the demonstrators smashed windows of the security offices outside the embassy before breaking through the main gate of the heavily fortified compound in eastern Sanaa. Security guards opened fire.

Film on al-Jazeera television showed demonstrators jumping up and down on the parapet of the building and scaling the walls.

The protests were triggered by an obscure video made in the United States that portrayed the Prophet Mohammad in insulting terms.

Yemen is fighting an al Qaeda-backed insurrection largely in the south of the country.