Armed tribesmen are battling government forces in a number of areas around the country, and Islamist militants, some linked to al-Qaida, have overrun entire towns in the country's restive south and are now fighting government forces and tribes that remain loyal to Saleh.

The botched airstrikes reflect the deteriorating security situation that has spread across the impoverished, heavily armed country since the popular uprising against longtime President Ali Abdullah Saleh began six months ago.

The president has clung to power despite the months of protests and being seriously wounded in an attack on his palace compound in the capital, Sanaa, on June 3. His wounds forced him to travel to Saudi Arabia for medical treatment, and he has yet to return. But Saleh has maintained his power through his son, who controls some of the country's best trained military forces.

The movement seeking an end to the president's 33 years in power got a boost Saturday with the announcement of a new tribal alliance whose leaders are pledging to defend the uprising.

Yemen's Tribal Alliance will group together one of the most powerful tribal confederations, the Hashid, and a number of tribes from the country's largest tribal confederation, the Bakil.

The Hashid leader already turned against Saleh in March, and fighters under his command have battled loyalist troops in the capital and elsewhere. "I pledge to you that Saleh will not rule us after today," said the Hashid leader, Sheik Sadeq al-Ahmar. He said the alliance will protect the protesters and provide security where government forces are failing to do so.

The U.S. and other nations worry that al-Qaida and other groups could exploit the turmoil in Yemen to step up operations. The airstrikes hit just east of the town of Zinjibar, near Yemen's south coast, which Islamist militants overran earlier this year. Since then, government forces and armed tribesmen have been battling to push them out, causing regular casualties on both sides.

Security official Abdullah al-Jadana said Saturday that men from the Fadl tribe advanced on Zinjibar, killing two militants and occupying a government communications building before at least three airstrikes hit the area late Friday, he said.

Tribal chief and field commander Mohammed Gaadani said the number killed in the airstrikes has risen to 40 people. He said his forces had notified the government of their locations and he condemned the strikes.

A military official confirmed the airstrikes and said preliminary information indicated a mistake had been made. He spoke on condition of anonymity in line with military rules.

Tribal loyalties are paramount in Yemen's provinces, where the central government exerts little control, and an errant airstrike could sap local support crucial to government forces.

Just north of the capital, Sanaa, two days of clashes with the army left 40 anti-government tribesmen dead, prompting their powerful tribe to threaten attacks against Sanaa's international airport.

Anti-government tribes in the mountainous Arhab region north of the airport have been battling Yemen's army for months. The tribes, which have long complained of neglect, say Republican Guard forces are shelling and bombing their villages, killing civilians.

The tribe has previously attacked local army bases and tried to prevent troops from entering the capital, where it feared they would attack protesters.

Sheik Hamid Assem said Saturday that two days of fighting had killed 40 tribesman. A military and a medical official said 28 soldiers were killed since the fighting began Thursday.

Late Friday, the Arhab tribe issued a statement warning it could strike the airport. "The sons of the Arhab tribe will strike the Sanaa International Airport with all the available means of war in response to the attacks on them by air and the shelling of their villages and homes," it said.

While the tribe likely cannot down an airliner, it could fire heavy weapons on the airport from mountains it controls nearby. It warned airlines not to use the airport "so that nothing bad happens to them."