The tribesmen have also occupied the national airline Yemenia building and have tried to storm the interior ministry headquarters, according to the witnesses and a high-ranking Yemen official.
The latest fighting came despite an appeal late Tuesday by President Ali Abdullah Saleh for supporters of Sheikh Sadiq al-Ahmar, who heads the powerful Hashid tribal federation, to "cease their aggression on security forces."
The president, who has warned the situation could deteriorate into civil war, also called on the tribesmen to "withdraw their armed partisans from public buildings and facilities," the defence ministry's 26sep.net news website said, adding that security forces would observe a ceasefire.
A number of residents were Wednesday seen fleeing Sanaa southwards, hoping to escape the fighting as well as electricity and water shortages, an AFP correspondent said.
Those attempting to head north ran into Republican Guards checkpoints and were advised that they may not be allowed to return to Sanaa. The area north of the capital is a stronghold of Ahmar's Hashid federation.
Clashes between security forces and Ahmar's followers broke out in the capital on Monday after Saleh refused to sign a deal with the opposition brokered by the Gulf Cooperation Council (GCC) that would see him leave office within 30 days.
The tribesmen occupied public buildings, including the trade and industry ministry, sparking fighting that raged through much of Monday and Tuesday.
After a brief overnight lull, gunbattles erupted once more on Wednesday morning.
According to an AFP tally based on reports by medics, the government and tribesmen, at least 44 people have been killed in the fighting.
Ahmar has accused Saleh of trying to spark a civil war in an attempt to remain in power.
Late Tuesday, a missile slammed into Ahmar's home causing casualties, a tribal source said.
"The home of Sheikh Sadiq al-Ahmar was hit by a missile and there were several dead or wounded," the source said.
"Among the wounded was General Ghaleb Gamash, who was leading a mediation mission."
The Yemen official warned, meanwhile, that the violence will escalate if the tribesmen don't evacuate the public buildings, which are located in or near the Al-Hasaba neighbourhood where Ahmar lives.
"Sheikh al-Ahmar's men are required to withdraw from the buildings under their control," he said, speaking on condition of anonymity. "Otherwise, we will force them to do so."
A Saba journalist said that tribesman had taken over state news agency's headquarters during the night.
"Armed men stormed the Saba headquarters during the night and demanded that we leave," the journalist told AFP on condition of anonymity.
Access to Al-Hasaba has been cut by cement blocks and burning tyres placed in the streets.
Ahmar in March pledged his support for the opposition, which since January, supported by widespread street protests inspired by popular uprisings in Tunisia and Egypt, has been demanding Saleh's ouster.
Yemen's wealthy Gulf neighbours on Tuesday demanded an immediate halt to the bloody clashes between regime supporters and Ahmar's clansmen.
"The fighting in Sanaa during the past two days is a source of concern for the GCC who fear that it may spread," said Gulf GCC Secretary General Abdullatif al-Zayani.
Sources close to Ahmar said the fighting had broken out on Monday after security forces tried to deploy around his residence and his gunmen retaliated.
A Yemen security official however said the gunmen broke into a nearby school and police responded.
The conflicting accounts could not be independently verified.
One of the 10 sons of Sheikh Abdullah al-Ahmar, who was until his death Saleh's main ally, Ahmar is capable of rallying thousands of armed supporters, tribal sources say.
Yemen has an estimated 60 million firearms in private hands, roughly three for every citizen.
The country's opposition vowed on Monday to step up street protests, while insisting on efforts to avoid violence.
Share
