SANAA, Yemen -- Thousands of armed tribesmen clashed with government troops in the mountains Thursday, preparing to march into Yemen's capital to reinforce their brethren in nearly two weeks of fighting that has pushed the impoverished country to the brink of civil war.
Artillery and gunbattles in Sanaa forced the closure of Yemen's main international airport, on the capital's outskirts. To the south, tribesmen attacked government forces in a second city, Taiz, highlighting how the Sanaa fighting threatens to flare around the highly fragmented nation, home to an active Al Qaeda branch.
Nearly four months of mostly peaceful street protests calling for democratic reforms and the end of President Ali Abdullah Saleh's 33-year rule have given way to an eruption of violence between Saleh's security forces and fighters loyal to Sheik Sadeq al-Ahmar, head of the country's most powerful tribal confederation, the Hashid, which has announced its support for the protest movement.
The move of new tribal forces toward Sanaa portended an expansion of a conflict that threatens to turn into an all-out battle for power. Given Yemen's complex web of tribal alliances and family rivalries, an increase in the tribal forces could suck in others of the many armed factions in a country rife with weapons.
Saleh's side was also stepping up its forces: Yemen's Defense 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 -- 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 took control of more than a dozen ministries and buildings since fighting broke out last week.
Fighting raged in at least two areas of the capital overnight, and in one of them -- the northern Hassaba district -- it continued into the day Thuyrsday. Heavy artillery and gunfire shook homes near the upper house of parliament, the central prison and the road to the state TV headquarters in the district, which has been the epicenter of the battles.
Sanaa airport was closed Wednesday night and remains shut for fear that planes could be hit in the shelling. Information on dead and wounded in Thursday's clashes was not immediately available, though residents said they'd seen many ambulances in the city.
Since the wave of protests against Saleh began in February, a number of army units have abandoned the president and announced their support for the opposition, including 1st Armored Division commander, Maj. Gen. Ali Mohsen al-Ahmar, who is not related to the tribal chief. So far, however, his 50,000 troops have not joined the fight, despite government attacks on their bases.
An al-Ahmar aide 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 authorized to speak to reporters.
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 powerful 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 are a formidable opponent to Saleh. The al-Ahmar family's 10 brothers play prominent roles in business and politics in Yemen, and several of them publicly criticized Saleh and called for his ouster well before the uprising. The Hashid confederation, made up of 10 northern-based tribes, is Yemen's most powerful tribal grouping.
A tribal leader, Mohammed al-Hamdani, said Thursday that several thousand Hashid fighters have moved out of the al-Ahmar family's ancestral homeland -- the city of Amran, 28 miles (45 kilometers) northwest of Sanaa -- and were camped out on the road to the capital, awaiting orders to move in to join their brethren. The fighters were organized into military-style units under the command of Hussein al-Ahmar, the younger brother of the tribal head, al-Hamdani said.
The fighters skirmished before dawn Thursday in hit-and-run attacks on members of the elite Republican Guard who had been pulled up to a military post on the highway near the camp to prevent any advance. Tanks and armored vehicles reinforced the post, and government warplanes swooped overhead with intimidating sonic booms.
A military official warned that the warplanes would strike the fighters if they tried to advance on the capital, a military official said. "There are orders and instruction that if they approach and engage, we can strike them with airplanes," the official said, speaking on condition of anonymity according to regulations.
For their part, the tribesman said they were awaiting orders to approach.
"We won't leave al-Ahmar alone and will enter Sanaa to stand with him and to fight alongside him," said al-Hamdani, the tribal leader.
South of the capital, tribal gunmen entered the city of Taiz, where anti-Saleh activists have held huge protests since the uprising's start and where security forces cracked down last week, occupying city streets and killing at least 25 people. The tribal fighters clashed with the security forces in fighting that killed at least three tribesmen and two soldiers, a security official said.
Security forces fired on a march in Taiz on Thursday morning and three protesters were wounded in fighting with soldiers and plainclothes security men, said activist Mohammed al-Darfi. Overnight, government troops stormed the home of Taiz opposition lawmaker Sultan al-Samie, confiscating his computer and documents.
The city's protest leaders 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 who had entered the city, she said, "We know nothing about them."