A suicide car bomber has killed at least six soldiers in Pakistan's North Waziristan district, officials say.
Tens of thousands of soldiers are deployed in the border area
At least two others were wounded when the bomber rammed his car into a checkpoint near the town of Miranshah, officials said.
The blast came a day after local militants in the area declared a month-long truce with security forces.
The BBC's Zaffar Abbas says such bombings in the Waziristan region are a major new challenge for authorities.
It is the third suicide attack in the area in the space of a month. Dozens of militants and troops have been killed in the area this year.
Tens of thousands of Pakistani security forces are trying to flush out foreign Islamic militants and their local supporters in the country's restive tribal belt along the border with Afghanistan.
"We can confirm at the moment that a car packed with explosives rammed a check post on the Bannu-Miranshah road," a government official told Reuters news agency.
Officials say the explosives-laden vehicle detonated about six kilometres (four miles) east of Miranshah, the main town in the North Waziristan region.
They say paramilitary soldiers at the checkpoint ordered the car to stop, and as it did so it exploded.
"The bomber's body was also blown to pieces," an official said.
Within a few minutes, helicopter gunships were in the air trying to locate other members of the group.
All roads in the area were closed off after the blast.
The casualties are the heaviest sustained by Pakistani security forces in North Waziristan since bloody clashes in Miranshah earlier this year.
The authorities have said it is unclear who is behind the attack.
Local pro-Taleban militants first denied involvement, and then told the BBC one of their members had detonated the explosives.
The militants' spokesman, Abdullah Farhad, claimed the bomber had acted in "self-defence" after security officials tried to arrest him.
He insisted the ceasefire the militants announced on Sunday still held.
Our correspondent says that the attack has inflicted a serious blow to efforts by the new governor of North West Frontier Province for a negotiated settlement of the conflict.
He says it was largely because of those efforts that local militants offered their one-month conditional ceasefire.
Officials say some of the demands - like the withdrawal of security forces from the area - were unreasonable, but some saw the offer as a positive first step towards a durable truce.