A U.S. drone killed four suspected al Qaeda militants on Monday, Jan 21 in a strike in central Yemen, tribal sources said, in the third such attack in three days, according to Reuters.

The United States never comments on strikes by its pilotless aircraft, which it has used to track down militants in Yemen for years. The Yemeni government tolerates such strikes but usually does not comment on the U.S. role in specific incidents.

Washington has scaled up action against al Qaeda in the strategically important state, where the group exploited widespread anti-government unrest in 2011 to seize swathes of territory in the south.

It was subsequently driven out by a military offensive in June last year.

Yemen's defense ministry said in a statement the air strike targeted a vehicle being used by militants in al-Ateef in the central province of Maarib, killing four fighters. It did not say who was responsible.

Tribal sources said a pilotless plane had attacked a car on a main road linking the capital Sanaa with Maarib.

Six suspected insurgents were killed in two other drone strikes in Maarib on Saturday and Sunday. A further 10 suspected al Qaeda fighters died in an explosion in a house in southern Yemen on Sunday.