A suicide bomber attacked an anti-al Qaida peace meeting attended by thousands of tribe members in north-western Pakistan, killing at least 40 people in the third suicide blast in the region in as many days.

Five tribes had called the meeting to finalise a resolution that would punish anyone who sheltered or helped Muslim militants, including al Qaida, Taliban and other foreign extremists, Interior Ministry spokesman Javed Iqbal Cheema said.

A young man walked up to a group of elders and blew himself up, said Alam Khan, a tribesman at the meeting.