PESHAWAR, Pakistan (AP) — A suicide bomber killed 23 people, many of them believed to be militants, in an attack today on the headquarters of a rival insurgent group in northwest Pakistan, a government official said.

The blast took place in the Tirah Valley region close to the border with Afghanistan, where clashes earlier in the day between Pakistani security forces and militants killed seven troops and 20 insurgents from an unknown group, security officials said.

The suicide bomber struck the base of the Lashkar- e- Islam group in the valley, said political administration official Iqbal Khan. He didn’t immediately know how many of those killed were militants and how many were civilians.

A commander of the Pakistani Taliban, which is a rival of Lashkar- e- Islam, claimed responsibility for the attack in a call to an Associated Press reporter. The commander gave his name as Mohammed.

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

Factional fighting among the insurgents is quite common in parts of the border region, where tribal loyalties hold sway and the government has little or no control.