Barack Obama has approved a new policy shift which allows the Central Intelligence Agency and the US military to launch drone attacks in Yemen when the identity of those who could be killed is not known.
Suspected al-Qaeda fighters can now be targeted based on lower-level intelligence.
But Human Rights Watch, the New York-based rights monitor, has strongly opposed the decision.
"We already don't know the scope of the drone ops in Yemen," the group said.
"The CIA is not a transparent organisation and international law requires a high degree of transparency when it comes to the killing of potential combatants."
Al Jazeera's Alan Fisher reports from Washington.