Pre-dawn street clashes between government forces and rival tribal fighters in the capital of Sanaa have killed at least 41 people Wednesday. Medical officials said the dead included combatants from both sides of the conflict.
The fighting spread to new areas, with tribesmen from the powerful Hasid confederation seizing buildings in neighborhoods in the city's south and northwest.
It also appears the military is no longer completely behind President Ali Abdullah Saleh. Witnesses said units of the elite Presidential Guard, commanded by one of Saleh's sons, shelled the headquarters of an army brigade responsible for guarding sensitive government institutions.
Army officers who have defected to the opposition said the government suspected the brigade commander was about to join forces with the movement to oust Saleh.
The urban battles over the last week have posed a new threat to Saleh's 33-year rule. For nearly four months, thousands of Yemenis have filled the streets daily, calling for democratic reforms and Saleh's ouster.
The mostly peaceful protests gave way last week to violence between Saleh's security forces and fighters loyal to Sheik Sadeq al-Ahmar, head of the country's largest tribal coalition.
The Associated Press contributed to this story