Fighting is worsening in the Yemeni capital as tribal fighters reportedly try to fight their way through government lines north of the city.
Witnesses say thousands more fighters are on their way to Sanaa and dozens of people are thought to have been killed.
President Ali Abdullah Saleh is refusing to step down despite months of opposition to his 33-year rule, the BBC reports.
Fighters loyal to a powerful tribal leader, Sheikh Sadiq al-Ahmar, have been battling government troops in Sanaa since last week.
He heads the Hashid tribal confederation that has declared its support for the Yemeni opposition movement.
A ceasefire broke down two days ago and street battles have raged since, leaving scores dead or injured.