The strikes were a partnered operation with the British, that official said. The officials spoke on condition of anonymity because operations were still going on.

The strikes marked a rapid escalation in tensions with Tehran and its proxy groups in Iraq, just two months after Iran carried out a massive ballistic missile attack against American troops at a base in Iraq. They came just hours after top U.S. defence leaders threatened retaliation for the Wednesday rocket attack, making clear that they knew who did it and that the attackers would be held accountable.

Defence Secretary Mark Esper told reporters at the Pentagon earlier Thursday that President Donald Trump had given him the authority to take whatever action he deemed necessary.

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

At the White House, Trump had also hinted that a U.S. counterpunch could be coming, telling reporters, «We’ll see what the response is.» And Army Gen. Mark Milley, chair of the Joint Chiefs of Staff, told Pentagon reporters the U.S. knows «with a high degree of certainty» who launched the attack.

On Capitol Hill earlier in the day, Marine Gen. Frank McKenzie, the top U.S. commander for the Middle East, told senators the deaths of U.S. and coalition troops created a «red line» for the U.S., but said he didn’t think Iran has «a good understanding of where our red line is.»

Asked if any counterattack could include a strike inside Iran, Esper said, «We are focused on the group that we believe perpetrated this in Iraq.»

Two U.S. troops and one British service member were killed and 14 other personnel were wounded when 18 rockets hit the base Wednesday. The U.S. military said the 107-millimetre Katyusha rockets were fired from a truck launcher that was found by Iraqi security forces near the base after the attack.

U.S. officials have not publicly said what group they believe launched the rocket attack, but Kataib Hezbollah, an Iranian-backed Shia militia group, was the likely perpetrator. And the U.S. strikes, which came in the middle of the night in Iraq, targeted that group.

Kataib Hezbollah was responsible for a late December rocket attack on a military base in Kirkuk that killed a U.S. contractor, prompting American military strikes in response.

That in turn led to protests at the U.S. Embassy in Baghdad. They were followed Jan. 3 by a U.S. airstrike that killed Iran’s most powerful military officer, Gen. Qassem Soleimani, and Abu Mahdi al-Muhandis, a leader of the Iran-backed militias in Iraq, of which Kataib Hezbollah is a member.

In response to the Soleimani killing, Iran launched a massive ballistic missile attack on Jan. 8, at al-Asad air base in Iraq, that resulted in traumatic brain injuries to more than 100 American troops.

Level of deterrence

McKenzie told the Senate armed services committee on Thursday morning that the killing of Soleimani and the increase in U.S. troops and assets in the region has made clear to Iran that the U.S. will defend its interests there. He said the U.S. has re-established a level of deterrence for state-on-state attacks by Iran.

However, he said: «What has not been changed is their continuing desire to operate through their proxies indirectly against us. That is a far more difficult area to deter.»

On Thursday, Esper and Milley said they spoke with their British counterparts about the attack, but declined to provide details.

Asked why none of the rockets was intercepted, Milley said there are no systems on the base capable of defending against that type of attack.

He also said the 14 injured personnel were a mix of U.S. and allied troops as well as contractors, and they will also be monitored for possible traumatic brain injury in the wake of the blasts.