In a statement on Thursday, Pentagon Spokesman John Kirby confirmed multiple structures belonging to the Iranian-backed Quds Force were destroyed in a USA airstrike.

The second European diplomat said USA leverage was still in place because President Joe Biden had not lifted sanctions.

Spokesman John Kirby said the operation targeted infrastructure in eastern Syria that were used by Iranian-backed militant groups.

"(Biden and al-Kadhimi) discussed the recent rocket attacks against Iraqi and coalition personnel and agreed that those responsible for such attacks must be held fully to account", the White House said in a statement of the two leaders' call on Tuesday.

But he also said that it was created to de-escalate the situation in eastern Syria and Iraq.

Earlier this month, rocket fire in northern Iraq killed a contractor working with the USA military and injured a US service member.

It comes as the USA tries to get Iran back into negotiations with a view to both countries re-entering the Iran nuclear deal.

Under the Trump administration, the escalatory back-and-forth stoked tensions, culminating in the USA killing of Iranian military leader Qassem Soleimani and a retaliatory Iranian ballistic missile attack against U.S. forces in Iraq a year ago.

Biden Administration officials condemned the February 15 rocket attack near the city of Irbil in Iraq's semi-autonomous Kurdish-run region, but as recently as this week officials indicated they had not determined for certain who carried it out.

On Monday, two rockets hit near the United States embassy in Baghdad, days after a volley hit an airbase further north where a U.S. military contractor is maintaining F-16 fighter-jets purchased from Washington. "We wanted to be sure of the connectivity and we wanted to be sure that we had the right targets".

The Pentagon had not blamed Iranian-backed militias for the attack in Erbil even though forensic evidence recovered soon after the attack pointed to a connection to Iranian-backed militias that have conducted similar attacks in the past. Experts say pro-Iranian Iraqi militant groups suspected of firing are seeking to increase pressure on their government amid Iran's impatience with the new United States administration.

Analysts and officials in Iraq say the resumption of attacks after four months of relative calm shows that Iran and its Iraqi allies are now abandoning de-escalation and seeking leverage over their rivals.

'Let's let the investigations complete and conclude, and then when we have more to say, we will'.

Although Kataeb Hezbollah did not claim responsibility for the attacks, US Defense Secretary Lloyd Austin said that the powerful pro-Iranian organization was behind them.

Trump had said the death of a USA contractor would be a red line and provoke United States escalation in Iraq.

Under Mr Trump, tensions with Iran soared, reaching a high point after a US-directed drone strike killed top Iranian general Qassem Soleimani.

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.