SANAA, Yemen, (AP) – Fighters for Yemen’s largest tribe sealed off key government buildings and barricaded streets in the heart of the capital Tuesday as the revolt against President Ali Abdullah Saleh sharply escalated after militiamen turned their guns against government forces.

The tribal fighters appeared to consolidate their control of a key district in Sanaa — which includes ministries and the ruling party headquarters — after the fiercest clashes in the three-month uprising against Saleh’s authoritarian rule. At least 12 people have been killed over two days of fighting, tribal chiefs and medical officials said.

The decision by Sheik Sadeq al-Ahmar — the head of the powerful Hashid tribe — to unleash his fighters sharply boosts pressure on the embattled president and suggests Yemen could be heading toward a potentially bloody showdown between well-armed tribal militias and pro-Saleh troops. Saleh has refused to step down despite three months of nearly nonstop street protests calling for an end to his 32-year rule.

In a clear blow to Saleh’s regime, tribal fighters carrying Kalashnikov rifles set up blockades around the Hassaba district, where buildings were pockmarked from gunfire and streets were flooded by water lines blasted by mortars.

The battles broke out after government forces tried to storm al-Ahmar’s house in the Hassaba area. After pushing back pro-Saleh forces, he warned the president to either call off his troops or face a harsher battle.

“We are exercising self-restraint,” al-Ahmar told The Associated Press. “We call on the regime to pull back its troops.”

Some military commanders have already defected to the opposition. But al-Ahmar’s move to join the battle could set Yemen on a dangerous path by forcing the other clan leaders around Yemen to pick sides and encouraging other military officials to abandon the government.

Yemen is a patchwork of tribes with strong militias whose support is critical for Saleh, who is also a member of al-Ahmar’s Hashid clan. In a bitter snub, al-Ahmar backed the uprising in March but held off sending his militiamen against government forces under an accord to keep the protests unarmed.

That changed Monday with the government attempt to storm his home. A senior military official in Sanaa told the AP that sending forces to al-Ahmar house was meant to “break the will of the tribes, but so far it’s failed.”

As the fighting raged, tribesman for the Hashid used locks and chains to seal off several important buildings, including the ruling party headquarters and the ministries for industry, economy and local administrations. Meanwhile, hundreds of lower-level tribal leaders vowed to camp out at al-Ahmar’s house in a show of solidarity.

“This is not an attack on al-Ahmar and his family only, but on all the tribes in Yemen,” said Faisal Manaa, a leader of the Bakeel, another powerful tribe. “We will not remain silent. We are warning the regime if it doesn’t withdraw its troops, we will be launching in a comprehensive and fierce war with them.”

On Tuesday, some mortars landed in the al-Ahmar compound and gunfire was heard in sections of the Hassaba district.

Dahan al-Qouhet, a tribal leader, said that at least five fighters were killed in the mortar strikes.

A medical official said that six al-Ahmar militiamen were killed and 37 others injured on Monday. The Interior Ministry said Saleh’s forces lost one soldier and five others were wounded.

The medical official spoke on condition of anonymity because he was not authorized to speak to the press.

The escalating clashes came after Saleh refused to sign a U.S.-backed deal, mediated by Gulf Arab neighbors, that offered immunity from prosecution under a timetable to step down within 30 days and transfer power to his vice president.

Although the tribal fighters seemed to tighten their grip in central Sanaa, government troops are still strong in other parts of the capital. Saleh’s forces — backed by his son’s well-trained and equipped Republican Guards — are concentrated in the southern part of the capital, where the presidential palace and military camps are located.