PESHAWAR, Pakistan -- A suicide bomber attacked the headquarters of a rival insurgent group in northwest Pakistan yesterday, 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 killed 41 other people, 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 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 in which militant groups are involved.

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 during the past year.

The Pakistani Taliban is one of that country'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 during the past four years. But terrorist attacks have continued. Within Pakistan's mostly Muslim population, there is considerable support or tolerance of extremist views, and the insurgents' anti-American rhetoric resonates loudly, hampering efforts to rally backing for the offensives.

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