A suspected U.S. drone fired a missile at a vehicle in Pakistan's volatile northwest on Monday, killing three people in the second such strike in as many days in an area dominated by militants who regularly attack U.S. and NATO troops in Afghanistan, officials and residents said.

President Barack Obama has stepped up the use of missile strikes in Pakistan's lawless tribal area since taking office, partly in response to the Pakistani government's reluctance to target Taliban militants who are not deemed a direct threat to the state.

The vehicle hit Monday was traveling through the North Waziristan tribal area, a region inhabited by militants who helped orchestrate the Dec. 30 homicide bombing against a remote CIA base in Afghanistan that killed seven of the agency's employees.

Yahya Khan, a schoolteacher in Tapi village where Monday's strike occurred, said he saw a drone fire a missile and went to inspect the damaged vehicle with one of his students.

"We saw the car had turned into pieces," said the student, Sabir Nawaz. "We saw at least two bodies."

Local government official Wazir Gul said three people were killed and he identified them as militants.

Pakistani intelligence officials provided conflicting accounts of the nature of the blast and the identities of those killed. Two officials confirmed the drone strike and said it killed three militants. Two others said the vehicle hit a land mine and could not confirm the identities of the three killed.

The officials spoke on condition of anonymity because they were not authorized to talk to the media.

The U.S. is thought to have launched more than a dozen missile strikes in North and South Waziristan since the CIA attack, including one on Jan. 14 that Pakistani and U.S. officials believe mortally wounded Pakistani Taliban leader Hakimullah Mehsud. A U.S. missile strike also killed Mehsud's predecessor in August.

The Taliban continue to deny Mehsud is dead but have failed to provide evidence he is alive.

The U.S. does not talk publicly about the secret CIA-run drone program in Pakistan, but officials say privately that the strikes have killed several senior Taliban and Al Qaeda leaders.

Pakistani officials regularly protest the strikes as violations of the country's sovereignty. But U.S. officials say privately that the government supports the program.