US-led coalition airstrikes are underway against multiple Iranian-backed militia sites in Iraq, according to a US official.

The strikes come one day after the US assessed an Iranian-backed group was responsible for a rocket attack on a base where coalition forces are located, killing two American service members and one British service member. Fox News was first to report on the strike.

There have been multiple rocket attacks in Iraq in recent weeks, but Wednesday’s was the first to cause a US death since December, when a US contractor was killed. That death prompted retaliatory US airstrikes against Iranian-backed militia targets in Iraq and Syria. Trump administration officials pointed to the contractor’s death as their justification for air strikes that killed Iran’s second most powerful leader, Gen. Qassem Soleimani.

The US was reviewing options as to how to respond, the US Defense secretary and top US general said earlier on Thursday.

“Yesterday’s attack by Iranian backed Shia militia groups consisted of multiple indirect fires that originated from a stationary platform and was clearly targeting coalition and partner forces on Camp Taji,” Secretary of Defense Mark Esper told reporters at the Pentagon while pledging that the US would take action to hold the attackers accountable.

“We’re going to take this one step at a time. But we’ve got to hold the perpetrators accountable. You don’t get to shoot at our bases and kill and wound Americans and get away with it,” Esper said.

President Donald Trump told reporters Thursday that he had been working on what to do about the attack on Wednesday night.

“They’ve sent a lot of rockets now. It hasn’t been fully determined it was Iran,” Trump said.

He added, “It was a rebel group, but most likely it looks like it could be backed by Iran.”

Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, said Thursday that the truck that fired the approximately 30 107mm rockets was captured by the Iraqi Security Forces and that forensics from that vehicle helped identify the attackers.

“We have pretty good confidence who did this. We were able to capture the truck, it was a modified truck with tubes, rocket tubes on the back, the Iraqi security forces did a good job in helping secure that truck. We have good indications based on forensics where it was fired from, who did the firing and so on and so forth,” Milley said.

Milley said that 14 coalition personnel were also wounded in the attack, including five injuries that were considered “urgent.” Those wounded included US, British, Polish and contractor personnel.

“The groups responsible will be held accountable appropriately,” Milley said, adding, “it was a significant attack and it resulted in the death and wounding of American (service members). So where we go from here as you would expect options are on the table and we are looking at everything and we will await final options and decisions from the President.”

The coalition strikes come one day after the House of Representatives approved a War Powers resolution aiming to rein in presidential authority to use military action against Iran without congressional approval.

The resolution — introduced by Sen. Tim Kaine, a Virginia Democrat — passed the Senate last month with bipartisan support despite Trump’s vocal opposition.

It will now be sent to the White House, where Trump is expected to veto it. Congress is not expected to have enough votes to override a presidential veto. Although it is unlikely to become law, passage of the resolution by both chambers represents a significant rebuke to the President and highlights congressional support for efforts to check the executive branch’s war-making powers.

This is a breaking story and will be updated.