|Protesters have faced bullets and tear gars while demanding an end to Saleh's 33-year-old rule [Reuters]
Yemeni security forces loyal to Ali Abdullah Saleh, the Yemeni president, have fired at protesters in Sanaa demanding an end to his rule.
Friday's violence in the Yemeni capital coincided with separate pitched street battles between security personnel - including government special forces originally set up to fight al-Qaeda - and tribal fighters trying to gain control of government buildings.
Street fighting in Sanaa, which grew out of protests against Saleh's rule since January, has killed at least 135 people in the past 10 days, calling into question the future of Yemen, an impoverished Arab state already near economic disaster.
Heavy fighting continued late into the night on Thursday, with witnesses saying security forces had fired live bullets at protesters - though no injuries were reported.
State television showed live pictures of the Yemeni Airways building ablaze and blamed tribesmen for setting it on fire. Al Jazeera has learned the fire started after government troops targeted the nearby home of Sadiq al-Ahmar, a prominent tribal leader.
Pro-Saleh forces have been fighting the powerful Hashed tribal confederation led by al-Ahmar with mortars, machineguns and rocket-propelled grenades for nearly two weeks.
Saleh has reneged on a deal brokered by the Gulf Arab states to secure a peaceful end to his nearly 33 years in power.
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Global powers worry that chaos in Yemen, home to al-Qaeda in the Arabian Peninsula (AQAP) and bordering the biggest oil exporter Saudi Arabia, would raise risks for world oil supplies.
Saleh's special forces were deployed to help "clean up" a ministry held by tribal forces, the defence ministry said, as battles near the airport briefly grounded flights.
Abdelqawi al-Qeysi, a spokesman for the Hashed tribal federation, said: "The weapons that America gave them to fight terrorism are being used against civilians."
State television showed heavily damaged government buildings which had been taken back by government forces.
The outside world has had little leverage on events in Yemen, where tribal allegiances are the most powerful element in a volatile social fabric, analysts said.
Saudi Arabia, which has strong, longstanding ties with Yemeni tribes, is likely to try to apply another round of pressure on Saleh to step down to avert disaster in a country of 23 million that is littered with guns.
Sanaa is split, with Saleh loyalists holding the south against tribesmen and renegade military units in the north.
Residents said dozens were likely to have been killed in the most recent round of fighting, mostly for control of government buildings and near the compounds of Saleh's tribal foes.
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Yemen is engulfed in multiple conflicts: in addition to the street fighting between security forces and Hashad tribesmen in Sanaa, there is popular unrest across the country and fighting against AQAP and other rebel fighters who seized the coastal city of Zinjibar.
Two soldiers were killed in an ambush on Thursday outside Zinjibar, a local official said, blaming the attack on rebels.
The UN human rights envoy said this week her office was investigating reports that Yemeni soldiers have killed at least 50 protesters in Taiz since Sunday.
Embassies have closed, diplomats have left and a refugee crisis may develop, with civilians fleeing Zinjibar and other towns. Oman has tightened patrols on its border with Yemen.
"The key risk for the region is either a prolonged stalemate [in Yemen] or a continued deterioration into a power vacuum," Christian Koch, director of international studies at the Gulf Research Centre in Dubai, said.
"This would accelerate the disintegration of the military institutions, intensify tribal conflicts and divisions, heighten the threat from extremist groups like AQAP by allowing them to spread their influence and possibly plan operations, and spark further separatist tendencies," he said.
Against this backdrop, Barack Obama's senior counterterrorism adviser arrived in the region on Wednesday to reinforce the drive to oust him.
John Brennan, the US envoy, left Saudi Arabia on Thursday for more talks on Yemen in the United Arab Emirates, a US official in Saudi Arabia said. He will seek the help of the two countries' leaders to pressure Saleh to accept the exit deal.
Later in the day, the Gulf Co-operation Council (GCC), whose power transition plan Saleh has refused to sign so far, said it would continue its efforts towards a peaceful solution.
"Abdulattif al-Zayani, the secretary-general of the GCC, confirms ... the GCC [continues] all efforts to help brothers in Yemen to reach a peaceful solution ending the ongoing fighting and to stop the bloodshed," a GCC statement said.