(Raises toll, adds details, witness account)

ISLAMABAD, March 2 (Reuters) - At least 39 people were killed and scores more injured when a suicide bomber attacked a traditional tribal meeting in northwestern Pakistan on Sunday, officials said.

Pakistan is in the middle of a wave of violence blamed on al Qaeda-linked militants based in tribal lands on the Afghan border and there have been three suicide attacks in as many days.

Over 500 people have been killed in militant related violence this year alone.

A top government official in Darra Adam Kheil tribal region told Reuters the bomber detonated a device while tribal elders were holding an outdoor "jirga", or traditional meeting.

"They were finalising the formation of a committee of locals to take steps against miscreants and help the government," said Kamran Zaib, a government official.

A security official who asked not to be identified put the number of dead at 39.

Local television showed pictures of residents and authorities cleaning up the blast site, a shady clearing surrounded by tall trees with a backdrop of rugged mountains.

Piles of torn clothing and bloody Muslim prayer caps were mixed up with the shattered remains of "charpoys", wood and rope daybeds.

"I saw three persons...all of them were not locals. The youngest one walked straight toward elders and blew himself up in the middle of them," said Naimat Khan, a witness.

Zaib said a head and identity card found at the scene were believed to belong to the bomber. He said the attacker was aged around 18-20.

A suicide attack on a police funeral in northwest Pakistan killed at least 38 people on Friday, while on Monday the army's top medical officer was killed in a bomb attack in Rawalpindi.

The escalating violence has raised concern about the stability of the nuclear armed state as it passes through a period of political transition, with doubts over how long President Pervez Musharraf can hold onto power after his allies lost a parliamentary election on Feb. 18.

Militants intensified their suicide bomb campaign after the army stormed Islamabad's Red Mosque last July to crush a militant student movement.

(Reporting and writing by Kamran Haider; Editing by David Fox)

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