SANAA: The deadly fighting that rocked the Yemeni capital this week spread beyond Sanaa on Friday as armed tribesmen seeking to oust President Ali Abdullah Saleh seized two military camps in battles that killed at least 18 and prompted airstrikes by government warplanes, a tribal leader said.

The fighting brought to at least 124 the number killed in the past five days of bloodshed, which has hiked fears that the Arab world's poorest country could be thrown into civil war as Saleh clings to power in the face of peaceful protests demanding his ouster.

There were new signs that the fighting in and around Sanaa could cause a wider breakdown in a country where numerous armed groups operate. On Friday, Islamic militants went on a rampage in a southern city, taking control of a police station, banks and government buildings, security officials and witnesses said.

This week's street battles in Sanaa have pitted Saleh's security forces and fighters from Yemen's most powerful tribal confederation, the Hashid, which has joined the popular uprising against the long-time ruler.

Friday's assault on the Republican Guard base in the Fardha Nehem region was the most significant escalation yet outside the capital. Tribal fighters allied to the Hashid stormed the camp, 50 miles northeast of Sanaa, and killed tens of troops — including the base commander — in the fighting, said local tribal leader Sheik Ali Saif.

After the fighters captured the camp, government airplanes bombed them and other forces clashed with them on the ground, he said. At least 18 tribesmen were killed, Saif said.

Tribal fighters then assaulted two helicopters that landed nearby, capturing them and a number of soldiers and shooting down a third helicopter, Saif said. Yemen's Interior Ministry denied the base's capture in a statement.

Saif said the tribe attacked the base to prevent its soldiers from heading to Sanaa to reinforce government troops there. The Republican Guard is one of Yemen's best trained and equipped forces. It is commanded by one of Saleh's sons and has remained loyal to the president even as other military units have defected.

Late Friday, tribal fighters seized another army base nearby after striking a deal with the soldiers inside that allowed them to leave with their personal weapons, local tribal leader Abdul-Moin al-Sharif said. No one was injured.

The week's fighting has terrified residents of the capital Sanaa, and many have packed up their cars and fled the city. Tens of thousand massed in the central square that has been the focus of the anti-Saleh protests for a march they said sought to "confirm the peacefulness of the revolution."

Addressing the crowd, Sheik Sadeq al-Ahmar, head of the Hashid, blamed Saleh for the week's violence.

"The war came down on our heads, but are holding strong and victorious," the 55-year-old leader said. "We want to remain peaceful, but if Ali Abdullah Saleh wants war, we are ready to face him and those around him."

For the first Friday since the uprising began, Saleh did not hold a boisterous rally for his supporters, apparently due to security concerns.

UN humanitarian chief Valerie Amos said she was "deeply alarmed by the escalating violence in Yemen" and called on all parties to protect civilians "and to spare them from the effect of further hostilities."

She cited reports of many families leaving Sanaa and expressed concern that "continued confrontations could force large numbers of people to flee their homes, with serious humanitarian consequences."

"Many Yemenis already face shortages of food, fuel and other basic necessities," Amos said in a statement. "The UN is monitoring the situation, and stands ready to offer assistance if needed."

The battles broke out Monday after an attempt by government forces to storm al-Ahmar's compound in the heart of Sanaa. By Thursday, the clashes had widened to include areas around Sanaa's airport, and other tribes had joined in alongside al-Ahmar's Hashid. On Friday, however, there were no reports of fighting in Sanaa, and Hashid gunmen solidified control over government buildings they'd seized, setting up checkpoints and searching those seeking to pass.

The fighting could open a new chapter in Yemen's turmoil. Until now, Saleh's opponents have stuck to peaceful protests massing hundreds of thousands around the country. Several military units, along with tribal powerhouses like the Hashid, have joined the opposition, but they have avoided violent confrontations with Saleh's loyalists.

But Saleh has managed to cling to power despite defections, protests and pressure from Arab neighbors and Western powers to leave office. Efforts to mediate his exit collapsed last week when the president refused to sign a deal for him to step down in 30 days.

Saleh has retained the loyalty of the regime's most elite military units — all commanded by close relatives.

The escalation with the tribes could strain that hold. Under Yemen's ancient codes, tribal leaders can declare that members follow their orders above all others — potentially forcing soldiers in pro-Saleh units to choose between their clan and military loyalties. So far, there have been no apparent signals of mass defections from the pro-Saleh units since the fighting with the Hashid began.

Most of Yemen's tribes boast heavily armed militias loyal to their chiefs. The northern-based tribes that make up the Hashid confederation hold powerful business and government interests. Yemen's other main tribal confederation, the southern-based Bakeel, is larger but has less political and economic power and, with many more tribes, is less cohesive. Most Bakeel tribes have turned against Saleh.

Also Friday, hundreds of Islamic militants seized control in Zinjibar, the capital of the southern province of Abyan, killing eight policemen and two civilians in gunfights with guards, security officials said. The fighters took over two banks, the city's tax bureau and two security offices. After the fight, the men could be seen driving freely around the city. A local army division made no effort to confront the militants, witnesses said.

Many fear militants will exploit Yemen's turmoil to take power. Officials said Friday's attackers were likely local Islamic militants who could be associated with al-Qaida. They spoke on condition of anonymity under government rules.

But Yemen has numerous Islamic militants who are not members of al-Qaida's branch in the country. Veterans of "jihad" in other countries like Iraq and Afghanistan, they have often been used by Saleh's regime to fight its opponents.

Ali Dahmis, a political analyst in Zinjibar, accused the regime of collusion in the assault on the city Friday.

"This is a theater produced by the regime to distract from the events in Sanaa and show that al-Qaida is a threat," he said.

The escalating violence has prompted the State Department to order nonessential U.S. diplomats and their families to leave the country. Britain said it would scale back its embassy staff, while Germany and other countries issued travel warnings.