At least four people were killed and some others were injured Friday when a U.S. drone fired missiles into northwest Pakistan's North Waziristan tribal region, local sources said.

Four missiles were fired from the pilotless aircraft at a house and a vehicle suspected of carrying militants in Datta Khel area of North Waziristan, a place bordering Afghanistan and believed to be a stronghold of militants, the sources said.

The latest strike is the fifth of its kind since the start of the new year. On Jan. 1, a string of four drone attacks in North Waziristan killed 19 people.

Since 2010, the United States has apparently stepped up drone strikes in North Waziristan, a Pakistani tribal region because the U.S. considers a launching pad for attacks on the foreign troops across the border into Afghanistan.

Pakistan publicly criticizes the U.S. drone strikes but it is widely believed that there is understanding between the two countries. The government denies any understanding.