The U.S. airstrikes on five bases in Iraq and Syria used by an Iranian-backed militia sent a warning to Tehran that President Donald Trump’s patience has its limits.

“One of the things that we want to emphasize is that this was a defensive action that was designed to protect American forces and American citizens in Iraq”, Brian Hook, the State Department’s Iran envoy, said in a telephone briefing Monday. “We’re also working on the mission set of restoring deterrence against Iranian aggression”.

Sunday’s rare direct strike on an Iranian proxy came at an especially tense time and held the potential for escalation. The U.S. and Iran are locked in a standoff over the Trump administration’s crippling economic offensive against Tehran – meant to force it to renegotiate the 2015 nuclear deal Washington has abandoned – and the Islamic Republic’s suspected reprisals.

Read more: Bloomberg