The statement said the strikes, conducted at President Joe Biden's behest, targeted multiple facilities used by Shia militant groups, including Kata'ib Hezbollah and Kata'ib Sayyid al-Shuhada.

The United States on Thursday carried out an airstrike in Syria against a structure belonging to what it said were Iran-backed militia, two officials told Reuters, an apparent response to rocket attacks against us targets in Iraq.

The assault came after a series of rocket attacks in recent days on facilities in Iraq used by the USA, including one that killed a contractor working with the US -led coalition in the country.

The Pentagon said the strikes were retaliation for a rocket attack in Iraq earlier this month that killed one civilian contractor and wounded a USA service member and other coalition troops. "At the same time, we have acted in a deliberate manner that aims to de-escalate the overall situation in both eastern Syria and Iraq", Kirby said. It wasn't immediately clear if there were any casualties in the USA strikes.

The rocket attacks on U.S. positions in Iraq were carried out as Washington and Tehran are looking for a way to return to the 2015 nuclear deal abandoned by former U.S. President Donald Trump.

Mary Ellen O'Connell, a professor at Notre Dame Law School, criticized the US attack as a violation of worldwide law.

Kirby did not say whether there were any casualties in Thursday's attack.

The deliberateness of the new administration's approach has raised questions both in Washington and in Baghdad about where Mr. Biden's red lines are when it comes to responding to attacks from Iranian-backed militias that target Americans in Iraq.

A week later, a rocket attack in Baghdad's Green Zone appeared to target the U.S. Embassy compound, but no one was hurt.

A little-known Shiite militant group calling itself Saraya Alwiya al-Dam, Arabic for Guardians of Blood Brigade, claimed responsibility for the February 15 attack.

US State Department spokesman Ned Price had said on Monday that the United States holds Iran responsible for the strikes.

A USA official, speaking on the condition of anonymity, said the decision to carry out these strikes was meant to send a signal that while the United States wanted to punish the militias, it did not want the situation to spiral into a bigger conflict.

Earlier this week, the Kata'ib Hezbollah group, one of the main Iran-aligned Iraqi militia groups, denied any role in the rocket attacks.

The frequency of attacks by Shiite militia groups against USA targets in Iraq diminished late past year ahead of Biden's inauguration.

The US struck eastern Syria, it claimed were being used by Iran-backed armed groups.

USA forces have been significantly reduced in Iraq to 2,500 personnel and no longer partake in combat missions with Iraqi forces in ongoing operations against the Islamic State group.