Clashes in the Yemeni capital escalated further on Friday, with the destruction of the headquarters of an opposition TV station.
Reports say the fighting in Sana'a is expanding into new neighborhoods of the city, and tribesmen loyal to the opposition are traveling to the capital city to take part in the fighting.
The rising chaos is pushing the conflict closer to an all-out civil war. Opposition groups say government troops have killed 50 opposition members in fighting this week.
Yemen is engulfed by multiple conflicts, with the street battles raging in Sana'a, popular unrest by anti-government demonstrators throughout the country and fighting against Islamist militants who have seized the southern city of Zinjibar.
In the southern city of Taiz, government forces and protesters clashed Thursday. At least 25 people have died in the violence in Taiz in the past few days.
Sana'a is split, with Yemeni security forces holding the southern part of the city against tribesmen and renegade military units in the north. The opposition is calling for the resignation of longtime President Ali Abdullah Saleh.
The clashes threatened to close the Sana'a airport, which lies 10 kilometers north of the city. The streets of the capital are now largely deserted, as thousands of residents flee to safer ground.
U.S. envoy John Brennan, President Obama's top counterterrorism adviser, left the U.S. Thursday to travel to the United Arab Emirates to continue talks on Yemen. He is seeking help to pressure President Saleh to accept a deal brokered by regional powers that would secure a peaceful end to his nearly 33-year rule.
The fighting in Sana'a broke out last week when pro-Saleh forces moved against the al-Ahmar compound in Hasaba, a district of the capital. In March, the al-Ahmar family had announced that the Hashid confederation - the country's most powerful tribal alliance - would back the protest movement, but its armed fighters had avoided clashes with Saleh's forces.