PESHAWAR - A suicide bomber attacked the headquarters of a rival insurgent group in northwest Pakistan on Friday, killing 23 of its supporters in an outbreak of factional fighting, an official said.

Elsewhere in the Afghan border region, fighting between security forces and militants claimed the lives of 41 others, an especially deadly day that showed the level of instability 10 years after the U.S.-led invasion of Afghanistan heralded the unrest.

Clashes among insurgents are quite common along the border, where tribal loyalties hold sway and the government has little or no control. Smuggling, kidnapping for ransom and drug production are all lucrative businesses that militant groups are involved in.

The suicide bomber struck the base of the Lashkar-e-Islam group in the Tirah valley, where Pakistani security forces are also fighting militants threatening the nearby city of Peshawar, said local political administration official Iqbal Khan.

A commander of the Pakistani Taliban, which is a local rival of Lashkar-e-Islam, claimed responsibility for the attack in a call to an Associated Press reporter. The commander, who gave his name as Mohammed, said it was in revenge for an attack last month that killed Taliban fighters.

The two groups have been fighting for control of the Tirah Valley over the last year.

The Pakistani Taliban is one of Pakistan's deadliest militant groups, and has links to al-Qaida and insurgents fighting across the border in Afghanistan. Its commanders are based in the border region, but the group has allied networks throughout the country.

Security forces have launched offensives in many border regions over the last four years, losing hundreds of men and killing thousands of alleged militants. But terrorist attacks have continued. Within Pakistan's mostly Muslim population, there is considerable support or tolerance of extremist views, and the insurgent's anti-American rhetoric resonates loudly, hampering efforts to rally backing for the offensives.

Earlier in the Tirah Valley, clashes between Pakistani security forces and militants killed seven troops and 20 insurgents from an unknown group, intelligence officials said. They didn't give their name because they were not authorized to speak to the media.

Elsewhere in the border region, Pakistani security forces backed by jets bombed two militant compounds in Orakzai, killing 14 insurgents and wounding 12 others, government administrator Javed Khan said. Security forces suffered no casualties, he said.

The border regions are off-limits to reporters and very remote, meaning it is hard to get detailed and independently verified information about events there. Militants and authorities often give differing accounts.