At least 29 people have been killed in a bombing near Swat valley in north-west Pakistan, officials say, report agencies..

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 ahead of an expected army assault against the militants in South Waziristan near the Afghan border.

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

The Pakistani Taliban said it was responsible for the attack in Rawalpindi, saying it had been carried out by 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. The latest attack took place early in the afternoon in the town of Alpurai.

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

Witnesses said area was strewn with debris. A military spokesman said several trucks were destroyed in the blast. Shangla district borders the Swat valley.

In June the army declared an anti-Taliban offensive in the Swat valley a success. But there have been isolated incidents of violence since then.

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

The Taliban says it was responsible for a weekend attack on the military headquarters in northern Pakistan which left at least 19 people dead.

Spokesman Azam Tariq said the attack was carried out by a Punjab faction of the group.

Armed militants ambushed the base in Rawalpindi, near Islamabad, taking hostages before being overpowered.

Azam Tariq said it was to avenge the recent killing of Pakistani Taliban leader Baitullah Mehsud by a US drone. In a telephone call to news agencies, the spokesman warned of further attacks.

"We will take revenge for our martyrs and will carry out more attacks, whether it's the GHQ (the army's general headquarters) or something bigger," he was quoted as telling Reuters.

In the wake of the assault, Pakistan's Interior Minister Rehman Malik said the military would "imminently" launch an operation in South Waziristan, site of strongholds of the Taliban and al-Qaeda.

Meanwhile: Pakistani aircraft attacked Taliban militants in their South Waziristan stronghold near the Afghan border as the government said a ground offensive against the al Qaeda-lnked fighters was imminent.

The aircraft struck the militants late Sunday, hours after commandos stormed an office building and rescued 39 people taken hostage after an attack on the army headquarters.

"The jets hit and destroyed two of their hideouts in Makeen and Ladha and we have a total of about 16 militants killed," a Pakistani intelligence official in the region said.

Pakistani Taliban militants linked to al Qaeda have launched numerous attacks on government and foreign targets over the past couple of years killing hundreds of people. The military has been conducting air and artillery strikes in south Waziristan for months, while moving troops, blockading the region and trying to split off militant factions.

But a ground offensive, in what could be the army's toughest test since militants turned on the state, has yet to begin.