ISLAMABAD: An unmanned American aircraft fired missiles at a vehicle in a Pakistani tribal area bordering Afghanistan Tuesday, killing five suspected militants and injuring two, Pakistani intelligence officials said.    

The strike was part of a spike in American drone attacks, which have drawn official Pakistani criticism in the past. The spurt of drone action threatened to add tensions to relations between the two allies, just as they appeared to be improving.

The latest drone strike came near Shnakhura village in North Waziristan, two officials said. They said the area of the drone attacks is dominated by anti-US militant commander Hafiz Gul Bahadur.

The officials spoke on condition of anonymity because they were not authorized to brief reporters.    Bahadur's group is known for frequent attacks on US troops in Afghanistan, but generally it does not carry out operations inside Pakistan.

Several recent drone strikes have killed militants affiliated with Bahadur's group. Tuesday's was the third drone attack in North Waziristan within three days. On Sunday, drone-fired missiles killed 10 suspected militants in two strikes 12 hours apart.

The covert CIA drone program has drawn strong criticism in Pakistan. Pakistani officials charge such strikes violate their country's sovereignty. Many Pakistanis complain that the strikes kill innocent civilians.