Pakistani intelligence officials say U.S. missile strikes have killed at least 15 militants in Pakistan's northwest tribal region.

Two of Wednesday's drone strikes hit a militant compound and a vehicle near Wana, the main town in the South Waziristan tribal area, along the Afghan border.

Officials say those killed are believed to have been allied with militant commander Maulvi Nazir, whose fighters are said to cross the border to attack U.S. and NATO troops in neighboring Afghanistan.

Later on Wednesday, a third suspected U.S. drone strike hit neighboring North Waziristan, killing at least five suspected militants. The tribal area is a known sanctuary for Taliban and al-Qaida-linked militants.

The United States has stepped up drone attacks since U.S. special forces killed al-Qaida leader Osama bin Laden on May 2 in the Pakistani city of Abbottabad.

U.S. officials have never publicly acknowledged the use of drones inside Pakistan, but have privately confirmed the strikes to various news outlets.

Pakistani leaders protest the attacks as a violation of the country's sovereignty, but Pakistani intelligence is believed to provide the U.S. information for the strikes.

U.S. officials say the missile strikes have severely weakened al-Qaida's leadership.

Separately, Pakistani officials say armed men in Baluchistan have set fire to two NATO trucks carrying oil to neighboring Afghanistan. No one was injured. The attackers were not captured.