SAN'A, 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 gun battles in San'a 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 San'a 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 Sheikh 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 San'a 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 best trained in Yemen's armed forces - were moving to "liberate" buildings in San'a seized by 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 - the northern Hassaba district - loud explosions continued to shake the area through the day Thursday. 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.