Thousands of tribesmen threatened to descend on Yemen's capital to join the battle against forces loyal to President Ali Abdullah Saleh as the country slid deeper into an all-out fight for power. Government forces in Sanaa unleashed some of the heaviest shelling yet against their tribal rivals in a dramatic escalation of the conflict on Thursday.
For months youth-led protesters have tried to drive out Saleh peacefully. But their campaign has been overtaken and transformed into an armed showdown between Yemen's two most powerful families, the president's and the al-Ahmar clan.
The al-Ahmar family heads the country's strongest tribal confederation, which has vowed to topple Saleh after 33 years in power.
Their nearly two week-old battle in Sanaa raises a dangerous new potential in Yemen: that tribal fighting could metastasise and spread across the impoverished nation. Tribes hold deep loyalty among Yemen's 25 million people, and the death of a member can easily draw relatives into a spiral of violence.
On Thursday, tribesman attacked security forces in the city of Taez, south of the capital, apparently to avenge deaths of protesters there last week or to protect them from new crackdowns. Saleh's security forces have cracked down hard on the street protesters, killing well over 100 since February, but until now tribal fighters had stayed out of the fray.
Thursday's attack suggests other tribes may see the fighting between Saleh and al-Ahmar as a sign it is time to get out their guns, as well.
Deeply worrisome to the United States in particular is the possibility Al-Qaeda's branch in Yemen -- one of the terror network's most active franchises in the world -- will exploit the chaos.
In Sanaa, Saleh's troops battered al-Ahmar's positions with some of the heaviest artillery bombardments of the conflict. Gun battles raged in the northern Hassaba district, the epicentre of the fighting that began 23 May and has since spread to other parts of the city. Troops set fire to the headquarters of a private TV station owned by one of the 10 al-Ahmar brothers.
Artillery and rocket fire also set alight the offices of a private domestic airline, several clothing warehouses and four houses close to the al-Ahmar family compound in the district.
There was no immediate word on deaths or casualties in the fighting, which has killed more than 160 people since it erupted.
The fighting forced Sanaa airport to close Wednesday night, but the defence ministry said in a statement it reopened Thursday and was operating normally. However, at least two flights to Sanaa were rerouted during the day to the southern port city of Aden, an Aden airport official said, speaking on condition of anonymity for fear of government reprisals.
The regime was also marshalling its forces: The defence ministry said for the first time in a statement that special forces units commanded by Saleh's son, Ahmed, had joined the fight.
The units, which are among the best equipped and trained in Yemen's armed forces, were moving to "liberate" buildings in Sanaa seized by al-Ahmar's fighters, who have overrun more than a dozen ministries and buildings since fighting began.
In the mountains outside the capital, several thousand fighters loyal to al-Ahmar were camped out on the highway to Sanaa, prepared to move in and join the battle, according to a tribal leader, Mohammed Al-Hamdani. They had moved out of the family's ancestral heartland -- the city of Amran, 28 miles (45 kilometres) northwest of Sanaa -- under the command of Hussein al-Ahmar, the younger brother of the head of the Hashid confederation, Sadeq al-Ahmar, he said.
"We won't leave al-Ahmar alone and will enter Sanaa to stand with him and to fight alongside him," al-Hamdani said.
Government reinforcements, including the elite Republican Guard backed with tanks and armoured vehicles, deployed to confront them, taking positions at a nearby military post.
The two sides skirmished before dawn Thursday in hit-and-run attacks, and government warplanes swooped overhead with intimidating sonic booms.
The warplanes will strike if the fighters try to advance on the capital, a military official warned, speaking on condition of anonymity according to regulations.
With a potentially decisive battle for the capital looming, the campaign to oust Saleh is increasingly taken out of the hands of those who launched it: the hundreds of thousands who have been holding daily protests in the capital and other cities. Led by youth activists, they had taken their inspiration from peaceful uprisings that toppled the leaders of Egypt and Tunisia.
In the capital, snipers opened fire on the Taghyeer, or Change, Square protest camp, the centre of the uprising, and wounded three people.
Now "the protesters are sort of the field on which these plays for power are being played out," said Gregory Johnsen, a Yemen expert with Princeton University.
The fighting in the capital broke out last week after government forces moved against the al-Ahmar family compound in Hassaba, a virtual armed fortress, like many compounds owned by Yemen's tribal chiefs. The al-Ahmar family had announced the Hashid confederation's backing for the protest movement weeks earlier, but its armed fighters had avoided clashes with Saleh's forces.
The Hashid -- which consists of around 10 northern-based tribes -- represent a formidable opponent. The al-Ahmar family's 10 brothers are among the country's richest businessmen and most powerful political figures. Their father, Abdullah, had long been in an uneasy alliance with Saleh that helped ensure the president's power. After his death, leadership of the Hashid was handed to his eldest son Sadeq, and ties with Saleh have steadily unravelled.
In recent years, Sadeq and his two strongest brothers, Hameed and Hussein, grew increasingly critical of Saleh and his management of the country. Their bitterness grew as Saleh put more and more power in the hands of his own family -- handing out security and government posts to his sons -- and his own, small tribe, the Sanhan.
The al-Ahmar are not necessarily looking to step into power themselves, said Johnsen.
"They don't need to hold the presidency themselves to increase their power," he said. "But their power will definitely grow if Salah falls."
So far, one other key player has stayed out of the fight. Maj. Gen. Ali Mohsen al-Ahmar, who commands the 1st Armoured Division, defected to join the opposition weeks ago but his 50,000 troops have not entered the fighting. The general has long had uneasy ties with the al-Ahmar family, to whom he is not related.
An aide to the general said Thursday the commander was avoiding confrontations.
"We know that Saleh wants to drag us into a war, but we will not engage in any military operations," the aide said, speaking on condition of anonymity because he was not authorised to speak to reporters.
Amid the manoeuvring by the country's power players, the youth activists leading the protests were trying to adhere to their strategy -- a peaceful campaign to bring democratic change.
In the southern city of Taez, where some of the biggest protests have taken place, organisers met Thursday to confirm their commitment to "peaceful protest and peaceful revolution until the regime falls," said activist Bushra Al-Muktari. As for the armed men clashing in the city with security forces, "we know nothing about them," she said.