MIR ALI, Pakistan -- Suspected U.S. drones fired missiles at militant targets in northwestern Pakistan on Monday, killing six people in the 15th such attack this month, said intelligence officials.

U.S. officials do not publicly acknowledge firing the missiles, much less comment on who they are targeting. It is unclear why the attacks have spiked.

They target Pakistan's border regions with Afghanistan -- home to al-Qaida terrorists plotting attacks on the West, insurgents battling the Pakistani government as well as militants behind attacks on NATO troops in Afghanistan.

On Monday, three missiles struck a house and vehicle linked to militants in a village near Mir Ali, a town in the North Waziristan tribal area that is under effective militant control, said the officials, speaking on condition of anonymity because they were not authorized to speak to the media.

Noor Khan, a resident in the village, said he saw drones in the sky before the strike.

Pakistani intelligence officials have said most of this month's strikes have targeted forces led by Jalaluddin Haqqani, an Afghan commander whose forces are one of the greatest threats to foreign troops in Afghanistan.

The U.S. has pressured Pakistan to launch a military offensive against the network, but Pakistan has not done so.

Many analysts believe the Pakistan army tolerates militants fighting in Afghanistan because they want to have a proxy group to maintain influence there after U.S.-led foreign forces withdraw.

Monday's strike was the 15th drone strike this month -- the most intense barrage since the strikes began in 2004. They have killed more than 71 people since Sept. 2, according to an Associated Press tally based on accounts by intelligence officials.

Pakistani officials often criticize the strikes as a violation of the country's sovereignty, but the government is widely believed to help the U.S. carry out the attacks. Allegations of civilian casualties in the attacks are not publicly investigated.