DERA ISMAIL KHAN, Pakistan >> American drone-fired missiles hit a vehicle traveling on the Pakistan side of the Afghan border on Tuesday, killing six suspected militants, Pakistani intelligence officials said.

The attack took place the Birmal district of South Waziristan, the officials said, speaking on condition of anonymity because they were not authorized to talk to the media.

The Pakistani army has tried to clear South Waziristan of militants, but they still have sanctuaries in the mountainous region.

Islamabad criticizes the drone strikes publicly but the government is widely believed to have supported the covert CIA-run program. That cooperation is believed to have come under strain as the U.S.-Pakistani relationship has deteriorated over the last year.

The strikes, which began in earnest in 2008, have killed scores of militants, including foreign al-Qaida members involved in plotting attacks on the West. There were more than 150 attacks in 2010, and less than half that number last year.

Tuesday’s attack was the tenth this year. U.S. official do not talk about the program.