Yemen: 'Thousands' of tribesmen march on Sanaa
Fighting is worsening in the Yemeni capital as tribal fighters reportedly try to fight their way through government lines north of the city.
Witnesses say thousands more fighters are on their way to Sanaa to boost the tribal forces.
Flights at Sanaa airport were disrupted due to the fighting, but officials say operations have returned to normal.
President Ali Abdullah Saleh is refusing to step down despite months of opposition to his 33-year rule.
Fighters loyal to powerful tribal leader Sheikh Sadiq al-Ahmar have been battling government troops in Sanaa since last week.
Sheikh Ahmar heads the Hashid tribal confederation that has declared its support for the Yemeni opposition movement.
A ceasefire agreed on Friday broke down two days ago and street battles have raged since, leaving scores dead or injured.
Reports on Thursday said armed tribesmen had left their ancestral home in the city of Amran 28 miles (45km) north-west of Sanaa to support their fellow fighters in the capital.
An advanced guard clashed with government troops in the northern outskirts of the city, tribal leaders said, and witnesses reported that some had broken through, bringing artillery with them.
A statement from the defence ministry claimed that the military had stopped the fighters from entering the city, but an army officer who defected said that fighting was continuing, AFP reports.
Correspondents say the situation in Sanaa now threatens to turn into an all-out battle for power.
Thousands of people have fled the city, while many shops have shut and there are long lines at petrol stations.
Resident Mohsen Sinan, 70, said he and his family were trying to flee Sanaa along with many other residents.
"Sanaa is deserted now and if these battles continue, Yemen will be finished," he said.
Anti-government protests in towns and cities across Yemen began about four months ago and have been largely peaceful.
However, recent fighting between tribesmen and government forces threatens to drag Yemen into civil war, analysts say.
Elsewhere in the country, government forces opened fire on protesters in the flashpoint southern city of Taiz.
Clashes took place near the presidential palace and a post held by the Republican Guard, an elite army unit loyal to President Saleh and led by his son Ahmed, according to AFP.
At least 50 people have been killed in Taiz since Sunday, according to the UN.
Some flights at Sanaa airport were diverted early on Thursday, but officials say it is now operating as normal. Last week the airport was also forced to close temporarily due to the conflict.
Meanwhile, US counterterrorism adviser John Brennan arrived in Saudi Arabia for talks on Yemen, which is also home to an offshoot of the al-Qaeda militant group known as AQAP.
The US has been urging President Saleh to sign a Gulf Co-operation Council-brokered deal that would see him hand over to his deputy in return for an amnesty from prosecution.
He has agreed to sign the deal on several occasions, but then backed out.
Sheikh Ahmar pledged his support in March to protesters who have been demanding Mr Saleh - who is also a member of the Hashid tribe - step down.