Thousands of armed tribesmen were headed towards Sanaa on Thursday to back their leader Sheikh Sadiq al-Ahmar, whose fighters are locked in deadly battles with Yemen's security forces, tribal leaders said.

The call for reinforcements follows heavy fighting between Ahmar's supporters and Yemen's security forces through the night in Sanaa's Al-Hasaba neighbourhood, the sheikh's stronghold, which killed at least 15 people, medics said.

Among the victims was a seven-year-old girl, who died of her wounds after she was hit by a stray bullet, said a medical official at Al-Jomhoreya hospital in Sanaa.

The tribesmen clashed with security forces at a military post 15 kilometres (nine miles) north of Yemen's capital, the sources said.

According to one tribal leader, the armed men "want to enter Sanaa to back their leader" Ahmar, who heads the powerful Hashid tribal federation.

Residents described the overnight clashes in Al-Hasaba as the "most violent" of the past two days.

Running street battles on Wednesday killed 47 people, medics said on Thursday, raising a previous toll of 39.

Fighting in the capital broke out on Tuesday after a truce collapsed between security forces and tribesmen who have taken control of public buildings across the capital.

The truce was announced May 27, after a week of fierce clashes that erupted when embattled President Ali Abdullah Saleh warned of a civil war as he refused to sign a Gulf-brokered plan for him to give up office as demanded by pro-democracy protesters.

Ahmar had in March pledged his support for protesters who have been demonstrating since January for the departure of Saleh, who has been in power since 1978.

The defence ministry's 26sep.net news website said tribesmen had on Wednesday occupied a building near the presidential palace, in the south of Sanaa.

Witnesses said thousands of people have fled Sanaa while many shops remained shut and there were long lines at petrol stations.

The witnesses said reinforcements from the Republican Guards, an elite unit loyal to the president, had been sent to Al-Hasaba.

A fourth army brigade camp located near the state television and radio headquarters was targeted by rockets, as was the interior ministry headquarters, witnesses said.

26sep.net, meanwhile, said government forces "regained control of a number of public buildings," without specifying which ones.

The website had said on Tuesday that Ahmar's tribesmen had seized both the headquarters of the ruling General People's Congress and the main offices of the water utility.

Saleh's government had accused Ahmar's fighters of breaking the truce, but sources close to Ahmar said Saleh's forces were to blame as they had opened fire on the tribal leader's compound.

US Secretary of State Hillary Clinton said Wednesday Yemen's conflict will not end unless Saleh and his government make way for the opposition to begin a political transition.

"We cannot expect this conflict to end unless President Saleh and his government move out of the way to permit the opposition and civil society to begin a transition to political and economic reform," the chief US diplomat said.

And in a potential for a further escalation of violence in the Arabian Peninsula country, previously unarmed protesters have resorted to carrying weapons in the flashpoint city of Taez, where they clashed Thursday with security forces, witnesses told AFP.

The witnesses said the clashes took place near the presidential palace and near a post held by the Republican Guard, an elite army unit loyal to the embattled Saleh and led by his son Ahmed.

No further details were immediately available.

Protesters who have turned out in their tens of thousands since late January to demand Saleh's departure have generally staged peaceful demonstrations which have inevitably been dispersed with violence by security forces.

According to an AFP tally based on medics' reports, more than 180 protesters have been killed since January and thousands wounded.