The US has launched air strikes in Iraq targeting the Iranian-backed militia believed to be responsible for the rocket attack in Iraq that killed and wounded American and British troops.
Lance Corporal Brodie Gillon, who served as a Reserve with the Scottish and North Irish Yeomanry, died after a dozen missiles were fired at Camp Taji, north of Baghdad.
Two US servicemen were also killed in the attack on Wednesday, and another 12 military personnel were injured.
A US official said multiple strikes targeted Kataib Hezbollah weapons facilities inside Iraq.
The strikes were a joint operation with the British, the official said.
We're going to take this one step at a time, but we've got to hold the perpetrators accountable.Mark Esper
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 another base in Iraq.
They came just hours after top US defence officials threatened retaliation for the Wednesday rocket attack, making clear that they knew who did it and that the attackers would be held accountable.
Officials said Karbala Province and Jurf al Sakhar were two of the locations.
Defence Secretary Mark Esper told reporters at the Pentagon earlier on 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,” Mr 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, Mr Trump also hinted that a US counter-punch could be coming, telling reporters: “We’ll see what the response is.”
Mark Milley, chairman of the Joint Chiefs of Staff, told Pentagon reporters the US knows “with a high degree of certainty” who launched the attack.
On Capitol Hill earlier in the day, Frank McKenzie, the top US commander for the Middle East, told senators the deaths of US and coalition troops created a “red line” for the US but said he did not think Iran has “a good understanding of where our red line is”.
Asked if any counterattack could include a strike inside Iran, Mr Esper said: “We are focused on the group that we believe perpetrated this in Iraq.”
The US military said the 107mm Katyusha rockets were fired from a truck launcher that was found by Iraqi security forces near the base after the attack.
Kataib Hezbollah was responsible for a late December rocket attack on a military base in Kirkuk that killed a US contractor, prompting American military strikes in response.
That in turn led to protests at the US Embassy in Baghdad. They were followed on January 3 by a US air strike that killed Iran’s most powerful military officer, General 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 January 8, at al-Asad air base in Iraq, that resulted in traumatic brain injuries to more than 100 American troops.