"The provocative behavior has been out there for months... So do I think they may do something? Yes. And they will likely regret it," Defense Secretary Mark Esper told reporters.

"We are prepared to exercise self-defense, and we are prepared to deter further bad behavior from these groups, all of which are sponsored, directed and resourced by Iran."

Several thousand Iraqi protesters attacked the US embassy in Baghdad on Tuesday, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters.