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

Two missiles were fired from the pilotless aircraft on a vehicle suspected of carrying militants in the 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 eighth of its kind since the start of this year. On Jan. 1, a string of four drone attacks in North Waziristan killed about 19 people.

In a rare show of strength, thousands of tribesmen on Friday marched in Miranshah, the center of North Waziristan, against the U.S. drone strikes, which they said killed innocent people and disrupted their lives.

Since 2010, the United States has apparently stepped up drone strikes in North Waziristan, a Pakistani tribal region 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 Pakistani government denies any understanding.