Iran has made a concerted effort to rein in militias in Iraq and Syria after the United States retaliated with a series of airstrikes for the killing of three U.S. Army reservists this month.

Initially, there were regional concerns that the tit-for-tat violence would lead to an escalation of the Middle East conflict. But since the Feb. 2 U.S. strikes, U.S. officials say, there have been no attacks by Iran-backed militias on U.S. bases in Iraq and only two minor ones in Syria.

Before then, the U.S. military logged at least 170 attacks against U.S. troops in four months, Pentagon officials said.