SANAA - Forces loyal to President Ali Abdullah Saleh battled with tribal fighters in Yemen's capital on Thursday in clashes that killed dozens as a US envoy flew around the region to try to stop a looming civil war.

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 an impoverished Arab state already near economic disaster.

Heavy fighting continued late into the night, with witnessing saying security forces had fired live bullets at protesters.

State television showed live pictures of the Yemeni Airways building ablaze and blamed tribesmen for setting it on fire. Al Jazeera television said the fire started after government troops targeted the nearby home of a tribal leader.

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Saleh has reneged on a deal brokered by the Gulf Arab states to secure a peaceful end to his nearly 33 years in power. US President Barack Obama's top counter-terrorism adviser arrived in the region on Wednesday to reinforce the drive to oust him.

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 aside to avert disaster in a country of 23 million that is awash with guns.

US envoy John Brennan 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.

Late on Thursday, the Gulf Cooperation 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.

DIVIDED CAPITAL, FRACTURED COUNTRY

Yemen is engulfed in multiple conflicts, with street battles between tribal groups and Saleh's forces in Sanaa, popular unrest across the country and fighting against AQAP and other Islamist militants who seized the coastal city of Zinjibar.

On Thursday, two soldiers were killed in an ambush outside Zinjibar, a local official said, blaming the attack on militants.

One constant factor is Yemen's crippling poverty. Jobs and food are scarce, corruption is rampant and about 40 percent of the population struggles to live on less than $2 a day.

In Sanaa, pro-Saleh forces have been fighting the powerful Hashed tribal confederation led by Sadeq al-Ahmar, with mortars, machineguns and rocket-propelled grenades for nearly two weeks.

The capital 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.

Civilians have been fleeing the city in the thousands, hurriedly packing possessions into their cars.

"It felt as if the artillery shells were flying next to my head ... My wife, my daughter were screaming. It was horrible," resident Sadeq al-Lahbe said before leaving.

"There is no electricity, no water and violent strikes shaking the house. Is this life?"

Security forces raided offices used by the opposition Suhail TV and confiscated equipment, an opposition official said.

The state news agency Saba said several gunmen were arrested at the office, which it said was used to attack nearby areas in the Hasaba district, the scene of intense street clashes.

In Taiz, about 200 km (120 miles) south of Sanaa, Yemeni soldiers fired warning shots at demonstrators protesting against a government they blame for bringing Yemen to the brink of ruin.

Troops later closed off Taiz in an apparent attempt to prevent people from joining planned anti-government demonstrations on Friday, travellers said.

Four elite troops were injured, medical sources said, in an attack officials blamed on the opposition alliance.

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," said Christian Koch, director of international studies at the Gulf Research Center in Dubai.

"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.