President Joe Biden is "sending an unambiguous message that he's going to act to protect Americans and when threats are posed he has the right to take an action at the time and the manner of his choosing," Press Secretary Jen Psaki said.

The Pentagon said Thursday's strike, which according to a Syrian war monitoring group killed 22 militia members, was in response for a series of rocket attacks targeting US soldiers in Iraq.

One of those strikes, on a military complex in the Kurdish regional capital Arbil on February 15, killed a civilian and a foreign contractor working with coalition forces, and wounded several US contractors and a soldier.

Psaki said the decision behind the strike was "deliberative" and that Biden's aim was for "deescalating activity in both Syria and Iraq."

Addressing criticism from some in Congress that Biden should have sought legislators' authority before ordering the strike, Psaki said "there was a thorough legal process and review in advance."