MIRANSHAH, Pakistan (Reuters) - Two U.S. drone attacks killed 18 militants in Pakistan on Saturday, intelligence officials said, after recent NATO incursions raised tensions with an ally critical to Washington’s war effort in Afghanistan.

The United States has escalated pilotless drone aircraft missile strikes against al Qaeda-linked militants in Pakistan’s northwest, with 21 attacks in September alone, the highest number in a single month on record.

Angered by repeated incursions by NATO helicopters over the past week, Pakistan blocked a supply route for coalition troops in Afghanistan after one such strike killed three Pakistani soldiers on Thursday in the northwestern Kurram region.

Taliban militants threatened to attack more tankers carrying fuel to Afghanistan over that route after three dozen of the vehicles were set on fire in south Pakistan on Friday.

The United States needs Pakistan as it scrambles to contain a raging Taliban insurgency in Afghanistan before U.S. troops start withdrawing in July 2011.

Border incursions and disruptions in NATO supplies underline growing tensions in the relationship.

On Saturday, two drone attacks within hours of each other killed 18 militants in Datta Khel town in North Waziristan tribal region along the Afghan border, intelligence officials said.

“In the first attack two missiles were fired at a house while in the second attack four missiles targeted a house and a vehicle. The death toll in the two attacks reached 18,” said one intelligence official. At least six foreigners were killed in the first strike.

There was no independent confirmation of the attacks and militants often dispute official death tolls.

A large number of Arab, Chechan and Central Asian insurgents have taken shelter with Pakistani militants in the lawless tribal regions on the Afghan border after fleeing the U.S.-led war in Afghanistan in late 2001.

U.S. officials say drones are valuable weapons which have killed high-profile Taliban and al Qaeda figures in an area in northwest Pakistan described as a global hub for militants.

Pakistan worries the strikes undermine efforts to deal with militancy because civilian casualties inflame public anger and bolster support for the fighters.

Elimination of high-profile targets could not be possible without Pakistani intelligence, however, analysts say.

Homegrown Taliban fighters continue to carry out attacks including suicide bombings despite a series of military offensives officials say have weakened them.

A Taliban spokesman, Azam Tariq, told Reuters by telephone from an undisclosed location militants had attacked the NATO supply trucks in southern Sindh province on Friday to avenge NATO incursions.

The al Qaeda-linked group would carry out more attacks on tankers on all roads used to transport NATO supplies, not just the one closed by Pakistani authorities.