Many people were feared to have been injured in Friday's attack

At least 40 people have been killed after a suspected suicide bomb attack at a volleyball pitch in the troubled north-west of Pakistan, officials say.

Officials said the bomber drove a vehicle onto the field as people gathered to watch a match. Emergency workers put the toll as high as 70.

The attack took place in Lakki Marwat, close to North and South Waziristan.

The Pakistani army has been conducting a campaign against the Taliban in the tribal areas since October.

Dozens of people were reported to be injured in Friday's attack. Several buildings collapsed, trapping people under rubble.

"The villagers were watching the match between the two village teams when the bomber rashly drove his double-cabin pick-up vehicle into them and blew it up," district police chief Mohammad Ayub Khan told AFP news agency.

'Soft' target

Mr Khan told reporters the attack may have been in retaliation for attempts by locals to expel militants.

"The locality has been a hub of militants," he said.

"Locals set up a militia and expelled the militants from this area. This attack seems to be a reaction to their expulsion."

The BBC's Aleem Maqbool reports from Islamabad that among those killed were members of a local peace committee who have been campaigning for an end to the violence.

Mushtaq Marwat, a member of the group, told Pakistan's Geo TV that the attack occurred as the committee was meeting in a nearby mosque.

"Suddenly there was a huge blast. We went out and saw bodies and injured people everywhere," he said.

Some initial reports about the attack said the vehicle that exploded was stationary, or that a bomber had walked towards the volleyball pitch.

North and South Waziristan form a lethal militant belt from where insurgents have launched attacks across north-west Pakistan as well as into parts of eastern Afghanistan.

Our correspondent says it had been feared that while the army was congratulating itself on its campaign, militants had simply escaped to neighbouring areas such as the one where Friday's attack happened.

The number of people killed in militant attacks in Pakistan is fast approaching 600 in just three months, with no apparent end to the violence in sight, he adds.

Militants have attacked both "hard" targets, including army or intelligence offices, and "soft" ones such as markets or the crowd that was hit in Friday's bombing.

The attack came as a general strike was held in Karachi, Pakistan's commercial capital, in protest against a bombing there on Monday and riots that followed.

The bombing, which killed at least 43 people, targeted a Shia Muslim march and was claimed by the Taliban.