Updated

With chains and locks, a powerful Yemeni tribe has sealed several government buildings as it lays siege to a vital Sanaa district in fierce clashes with government forces.

A medical official says Tuesday that a total of seven people have been killed and 42 others injured since fighters of the Hashid, Yemen's biggest tribe, have engaged in an unprecedented confrontation since Monday with troops of President Ali Abdullah Saleh. The official spoke on condition of anonymity because he was not authorized to speak to the press.

Saleh himself belongs to the Hashid, but its leader, Sheik Sadeq al-Ahmar, has announced he is joining the uprising to try to oust Saleh.

Automatic weapons, mortars, and tanks are being used to blast some buildings, setting government offices on fire.

By the end of Monday, Hashid militiamen appear to be largely in control of the districts around the ministries of industry and economy — and even the ruling party headquarters.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

SANAA, Yemen (AP) — Security forces and opposition tribal fighters battled with automatic weapons, mortars and tanks in the Yemeni capital on Monday, blasting buildings and setting government offices on fire in violence that hiked fears of an armed confrontation after the collapse of efforts to negotiate a peaceful exit for President Ali Abdullah Saleh.

The street fighting, in which six people were killed and nearly 40 wounded, was the heaviest clash between the pro- and anti-Saleh camps since hundreds of thousands of Yemenis began taking to the streets three months ago in protests demanding the ouster of the president after 32 years in power.

It erupted amid increased tensions after Saleh refused at the last minute on Sunday to sign a U.S.-backed agreement, mediated by Yemen's powerful Gulf Arab neighbors, under which he would step down in 30 days. Saleh had promised to sign the deal, but instead, his regime sent mobs of armed supporters into the street Sunday, protesting at embassies, in an orchestrated campaign to demand he stay in power.

The United States expressed growing frustration with Saleh, an ally that Washington has relied on to fight al-Qaida's branch in the impoverished nation at the southern tip of the Arabian Peninsula. Fearing the turmoil is disrupting the campaign against the terror group, Washington has been trying to manage a transition that will keep some measure of stability.

Secretary of State Hillary Rodham Clinton said the Obama administration is "deeply disappointed" by Saleh's refusal to sign the accord, saying the Yemeni leader "is turning his back on his commitments and disregarding the legitimate aspirations of the Yemeni people."

So far both sides in Yemen's turmoil have tried to avoid a direct armed confrontation. The protesters are backed by heavily armed tribes and by army units that defected to the opposition, while Saleh has been able to cling to power by retaining the loyalty of the country's best trained and equipped military and security forces, which are headed by his relatives. His security forces have cracked down on protesters — killing at least 150 over the past three months — but the two sides' armed factions have generally eyed each other warily around the capital without major clashes.

But Monday's fighting underlined how explosive the tensions could be.

The violence erupted outside the Sanaa home of Sheik Sadeq al-Ahmar, leader of Yemen's largest and most powerful tribe, the Hashid. Saleh himself belongs the tribe, but al-Ahmar announced in March that the Hashid were joining the popular uprising against the president. Fighting raged for more than six hours, until the U.S. ambassador mediated a cease-fire, according to a ruling party official.

Witnesses said security forces had been setting up roadblocks between the walled compound and the nearby Interior Ministry, and that tribesmen saw it as a provocation. Abdel-Qawi al-Qawsi, an aide to al-Ahmar, accused security forces of trying to storm the Hashid leader's residential compound and said tribal fighters counterattacked.

The Hassaba district, where the al-Ahmar compound and a number of ministries are located, was turned into a battle zone, as tribesmen and security forces battled in the streets outside the Interior Ministry, trading fire with automatic weapons and rocket-propelled grenades. An office building of Yemeni Airlines was on fire, and smoke poured out of a building inside the Interior Ministry compound.

Tanks were seen pulling into the neighborhood and the sound of tank fire was heard. Hashid fighters outside Sanaa rushed into the capital to reinforce their comrades, one tribal official said, speaking on condition of anonymity because of the sensitivity of the situation.

Some security forces took refuge in the nearby headquarters of the state news agency Saba, and tribal fighters who seized the Industry Ministry across the street opened fire on them with mortars and automatic weapons, said Hassan al-Warith, Saba's deputy editor-in-chief. The news agency's staff took refuge in the basement, and one journalist was wounded by flying shrapnel.

"We have been trapped here in the basement. The top floors have been destroyed," al-Warith said, pleading for the authorities to rescue them.

The gunfire ceased in the evening, but fighters and security forces remained in the district.

A member of al-Ahmar's family said five tribal fighters were killed and 37 other fighters wounded. A bystander was also killed and two others wounded, according to a security official. The family member and the security official spoke on condition of anonymity because they were not authorized to speak to the press.

Yemen's tribes are heavily armed, and their fighters virtually are militias loyal to the tribal leader. Al-Ahmar's residence, overlooking the main road to the airport in Sanaa, is sprawling, fortified compound, surrounded by walls and barricades on nearby streets. Often, armed tribesmen are seen on rooftops nearby keeping guard.

Saleh and the opposition have traded accusations that each is leading the country into civil war.

But the beleaguered president faces increasing impatience from the United States after his last minute balking at the accord. Saleh has backed out of promises to sign the deal twice before, each time coming up with new excuses. A coalition of opposition parties signed the agreement on Saturday, and the president had promised to do the same on Sunday — but suddenly demanded that the opposition join him in a public signing ceremony.

Amid Sunday's turmoil, armed Saleh loyalists trapped the U.S. ambassador along with European and Arab ambassadors for hours inside the United Arab Emirates Embassy. The American ambassador was taken by car out of the mission to the presidential palace to witness while a number of top ruling party officials signed the accord — but Saleh refused.

"We've seen all sides agree on multiple occasions to sign their initiative. And now it appears that President Saleh is the only party that refuses to match his actions to these words," State Department spokesman Mark Toner told reporters in Washington.

"We believe that President Saleh still has the ability and the opportunity to sign this initiative and break this deadlock," Toner said. "But, you know, it's clearly now in President Saleh's hands."

In a sign of the tensions, a protest activist said an anti-Saleh army unit seized two supporters of the president who were allegedly storing weapons — apparently for use in attacking protesters — near Taghyeer, or Change, Square, the roundabout in the capital that has been the epicenter of the protest movement, with thousands camping there for weeks.

A coalition of Yemeni opposition parties said the protesters would remain peaceful and accused the regime of "assaults that aim to drag the country into civil war."