The U. S. military on Thursday launched an airstrike on services in eastern Syria used through Iranian-backed militias in retaliation for rocket attacks on U. S. troop sites in Iraq, the Pentagon said.

“Under President Biden’s leadership, U. S. army forces carried out airstrikes tonight against infrastructure used by Iran-backed militant teams in eastern Syria,” spokesman John Kirthrough said in a statement.

“These movements were legal in reaction to the recent attacks on the United States and the coalition workers’ corps in Iraq, and the continuing threats against that body of workers,” he said.

The U. S. military said there were casualties in Thursday’s attack.

Kirthrough said the target is a border checkpoint used through Iraqi armed groups backed by Iran, adding Kataeb Hezbollah and Kataeb Sayyid al-Shuhada.

Three rocket attacks followed on services in Iraq used through U. S. forces and the coalition fighting the Islamic State group.

One of the movements against an army in the Kurdish capital Arbil on February 15 killed a civilian and a foreign contractor working with coalition forces and wounded several U. S. contractors and a soldier.

Attacks in Iraq through teams suspected of operating Iran’s leadership had challenged Biden’s new administration just as it opened the door to the resumption of negotiations with Tehran on its nuclear program.

Biden’s leadership says so to revive the 2015 agreement to freeze Iran’s nuclear development.

But he sees Tehran as a permanent security risk across the Middle East.

Kirby called Thursday’s measures “proportionate” and said they were “conducted with diplomatic action,” consultations with US partners in the anti-IS coalition.

But he also said it was designed to defuse the stage in eastern Syria and Iraq.

“The operation sends an unequivocal message that President Biden will act before U. S. and coalition staff,” he said.