British Foreign Minister Dominic Raab on Friday praised the US-led response to an attack on coalition forces in Iraq, calling it "swift, decisive and proportionate".
On Thursday, the United States carried out several air strikes against an Iran-backed militia in Iraq that it blamed for a rocket attack that killed two US troops and a British soldier, 26, the previous day.
The Pentagon confirmed the US had struck five Iran-backed militia weapons stores in Iraq.
The strikes were "defensive, proportional and in direct response to the threat posed by Iranian-backed militia groups", it said.
The Pentagon said US President Donald Trump had authorised the military to response to Wednesday's attack, blaming Iran-backed militia.
On Friday, Mr Raab warned that attacks on coalition forces will lead to a strong response.
"UK forces are in Iraq with Coalition partners to help the country counter terrorist activity and anyone seeking to harm them can expect a strong response," he said in a statement.
Defence Secretary Mark Esper and Army Gen Mark Milley, chairman of the Joint Chiefs of Staff, on Thursday warned that all options were on the table, suggesting the US, Iran and the forces Tehran backs were again on a path towards confrontation inside of Iraq.
"I have spoken with the president," Mr Esper said at the Pentagon.
"He's given me the authority to do what we need to do, consistent with his guidance."
The US has repeatedly and publicly warned that killing Americans overseas was a red line that would lead to a response.
"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."
Washington blamed Kataib Hezbollah for a strike in Iraq in December that killed a US contractor.
Retaliatory attacks led to the US killing top Iranian general Qassem Suleimani, and an answering Iranian missile attack that left more than 100 US troops with brain injuries.
The head of the US Central Command, Gen Kenneth McKenzie, said on Thursday that Iran's missile strikes were an escalation beyond earlier attacks on US forces.
"In early January, Iran launched more than a dozen ballistic missiles in a deliberate attack against US and coalition forces at two bases in Iraq. This state-sponsored missile strike crossed a threshold compared to previous attacks," Gen McKenzie said in a tweet posted on Centcom's Twitter account.
In Wednesday’s attack, about 14 US-led coalition personnel were wounded, including Americans, Britons, Poles and others. Private industry contractors were among the wounded.
Gen Milley said five of the wounded were "urgent", suggesting serious injuries.
Britain identified its fallen service member as Lance Cpl Brodie Gillon, 26, who was with the Irish Guards Battle Group.
The US has not yet identified its killed service members.
Gen McKenzie said only Iran-backed Kataib Hezbollah had been known to launch such an attack in the past.
"While we are still investigating the attack, I will note that the Iranian proxy group Kataib Hezbollah is the only group known to have previously conducted an indirect fire attack of this scale against US and coalition forces in Iraq," he told a US Senate hearing earlier on Thursday.
The US-led military coalition in Iraq says 18 107-millimetre Katyusha rockets struck Iraq's Taji military camp.
The Democratic-led US House of Representatives passed legislation on Wednesday to limit Mr Trump's ability to wage war against Iran.
The Republican president has been engaged in a maximum-pressure campaign of sanctions against Iran, after pulling the US out of the international nuclear deal signed by his Democratic predecessor, Barack Obama.
Tension between Washington and Tehran has mostly played out on Iraqi soil in recent months.
Iran-backed paramilitary groups have been regularly rocketing and shelling bases in Iraq that host US forces, and the area around the US embassy in Baghdad.