A U.S. drone killed four suspected al Qaeda militants on Tuesday in a strike on their car in northern Yemen as they drove away from a militant training ground, according to tribal sources and local officials, the fourth such attack in four days.

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 Yemen, where the group exploited widespread anti-government unrest in 2011 to seize swathes of territory in the south.