Hours later, a bomb exploded in Pakistan's southwestern province of Baluchistan, where authorities have been fighting an ethnic insurgency. The blast killed at least three people and injured 12 more.

The bomb - planted on a bicycle in Quetta, the capital of Baluchistan - exploded as a police van drove past. It killed two policemen and one civilian, hospital officials said. Hamid Shakil, a senior police official, said it was apparently triggered by remote control. He blamed the attack on Baluchi nationalists - a militant movement separate from the Taleban insurgency that has been fighting the central Pakistani government for years.

Separately, helicopter gunships pounded Taleban hide-outs in the Kurram and Orakzai tribal regions, killing 17 militants, officials said.

Intelligence officials, speaking on condition of anonymity because they were not authorized to talk to the media, said 10 militants were killed in Kurram and seven in Upper Orakzai.

Meanwhile, in the tribal region of North Waziristan, officials said the four dead tribesmen were kidnapped by the Taleban about 10 days ago.

Gul Akber Khan, who lives in the village of Srakhula, just outside of Mir Ali, said he heard gunshots in the middle of the night. When he went to the mosque for morning prayers a few hours later, he found the bodies dumped along the road into Miran Shah, the main city in North Waziristan.

Intelligence officials, also speaking on condition of anonymity, said a note was found by one of the corpses. It warned, in Pashto: "Spies are spies, and they will come to the same fate as these men. ... Do not spy for America." The tribal regions along the Pakistan-Afghanistan border, strongholds for Al-Qaeda and Taleban fighters, have seen bloody fighting and regular attacks by American drone aircraft as the Pakistani and US governments try to defeat the militants.