(For more on Pakistan & Afghanistan, click [ID:nAFPAK]) (Updates with 13 militants killed in clashes in Swat, U.S. Afghan commander meets Pakistani army chief)

By Faris Ali

PESHAWAR, Pakistan, Aug 17 (Reuters) - A bomb planted in a vehicle killed six people in northwestern Pakistan on Monday as soldiers killed 13 militants in fresh clashes in the Swat valley where the army has made gains in a three-month offensive.

There has been a lull in militant violence in Pakistan in recent weeks after security forces pushed back Taliban insurgents in their bastion of Swat and stepped up attacks on Pakistani Taliban in the South Waziristan region.

But Monday's blast and the clashes in Swat, which followed two suicide bombings there on the weekend that killed five soldiers, will raise fears that the militants are re-organising.

Three women and two children were among those killed when the bomb hidden in a box of medicine given to the vehicle's driver to deliver went off, police officer Sifwat Ghayyur told reporters at the site of the blast in Charssada town.

"It was a time-device which went off when the driver stopped to get petrol. Two women, three children and the driver were killed," Ghayyur said.

Taliban militants claimed responsibility.

"We did it because these people were from a tribe which raised a lashkar against us," a militant spokesman in the region, Qari Shakeel, said by telephone, referring to a pro-government militia.

The military went on the offensive against Pakistani Taliban in the Swat valley in the northwest in late April, which initially triggered more bomb attacks by militants.

The army has killed or driven out many militants from Swat in what has been widely seen as a successful operation.

The military push has allayed fears among Pakistan's allies, in particular the United States and other countries with troops in neighbouring Afghanistan, that nuclear-armed Pakistan was failing to get to grips with spreading Islamist violence.

"CURFEW IN PARTS OF SWAT"

Pakistani efforts to suppress militants on its side of the border are vital for a U.S.-led bid to stabilise Afghanistan, where the Taliban have threatened to disrupt Thursday's presidential election.

The commander of U.S. and NATO troops in Afghanistan, U.S. General Stanley McChrystal, met Pakistani army chief General Ashfaq Kayani, for talks at the Pakistani army headquarters, the military said, although it gave no details.

Bomb attacks have become more rare in Pakistan in recent weeks although the reason for that is not clear. No top Taliban leader has been killed or captured in the Swat offensive leading to fears they could try to stage a come-back.

A Taliban spokesman from the valley said his men would soon step up attacks.

"We stopped our activity for a few days but will resume during or after Ramadan," the spokesman, Muslim Khan, said by telephone, referring to the Muslim fasting month due to begin late this week.

The army said 13 militants were killed during searches in the valley, where earlier authorities re-imposed a daytime curfew in some areas to hunt for militants.

"We have some information that some militants are hiding on the outskirts of Mingora, and it's difficult to carry out a search when there are people around," said the spokesman Lieutenant Akhtar Abbas, in Swat's main town of Mingora.

The army says the valley has been largely secured but troops still encounter pockets of resistence. The fighting forced nearly 2 million people from their homes but many have returned. (Additional reporting by Alamgir Bitani, Junaid Khan and Izaz Shams; Writing by Kamran Haider; Editing by Robert Birsel)

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