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At least 41 people have been killed in a suspected suicide bombing in Pakistan's Swat valley, officials say.

The explosion hit a security convoy in Shangla district - an area the military said it had retaken from militants.

It is the latest in a string of attacks and comes amid warnings of an offensive against militants in nearby South Waziristan on the Afghan border.

On Saturday militants stormed the army headquarters in Rawalpindi. Pakistan vowed to hit back "imminently".

The Pakistani Taliban said it had carried out the attack in Rawalpindi, through a Punjab faction of the group.

Spokesman Azam Tariq said it was to avenge the recent killing of Pakistani Taliban leader Baitullah Mehsud by a US drone.

Dozens hurt

The latest attack took place early in the afternoon in the town of Alpurai, in Swat valley in Shangla district.

ANALYSIS Syed Shoaib Hasan, BBC News, Islamabad While most of Swat's territory, especially its urban areas, are firmly in the control of the army, recent attacks suggest that the militants are not completely routed. But that is something the military itself has admitted, saying that Swat's rural areas are still a source of trouble. However, they remain confident that the situation will soon be under control. This does leave a big question mark over the South Waziristan operation. While the political leadership has been talking about destroying the militants in their main stronghold, the military has firmly kept itself aloof from any such adventure. But with the militants striking back hard, a national consensus seems to be pushing them towards launching an all-out operation. With winter around the corner, the military will have to move soon if it aims to achieve this. At the moment though, the initiative lies with the militants.

Alpurai was not under Taliban control at the time of a major anti-Taliban offensive by the military in Swat valley earlier this year.

Security officials say a car blew up near a hospital in the market as a convoy of troops was passing by.

Dozens of people, including security personnel, are said to have been injured in the explosion.

"It appears to be a suicide attack," Reuters news agency quoted Shangla police official, Khan Bahadur Khan as saying.

"The bomber hit one of three military vehicles that were passing through the busiest market in the district," he said.

Witnesses said the area was strewn with debris. A military spokesman said several trucks were destroyed in the blast.

The bombing - the fourth major attack within a week - brings to over 100 the death toll in four days of militant attacks across Pakistan.

Pushed out

In June the army declared the three-month anti-Taliban offensive in the Swat valley a success.

t has arrested several high-profile Taliban leaders in Swat since then, including the spokesman for the Taliban in Swat, Muslim Khan.

Muslim Khan's capture was viewed as a major coup against the Taliban, although the group's leader in Swat, Maulana Fazlullah, remains at large.

More than a million people were displaced by the Swat offensive. Many have since returned and the army maintains a significant presence there.

Some analysts say that in the wake of the army onslaught a number of militants fled to neighbouring districts.

During the offensive, fighting also spilled into Shangla district.

Over the past few months troops have also been gathering on the border of South Waziristan, where the Taliban have one of their main strongholds.

After Baitullah Mehsud was killed by a US missile in early August, there was a relative lull in Taliban attacks.

But there has been a resurgence in militant activity since the start of this month.