The US Defense Secretary says America is not seeking a war with Iran, but is "prepared to finish one".
Mark Esper said the Trump administration is seeking diplomatic solutions to the heightened Gulf tensions, but is insisting Iran must "de-escalate".
Mr Esper also stressed US troops would not be pulled out of Iraq, despite a draft letter appearing to suggest troop removal was underway.
Meanwhile, UK Defence Secretary Ben Wallace said he is "not going to rule out anything" when questioned about the likelihood of a British military strike on Iran.
Mr Wallace went on to say "the UK will do what it has to do to defend its persons, its citizens and wherever it needs to do that".
"That is our duty," Mr Wallace said.
"We cannot say what is in the minds of Iran or anybody else in the future and that's why we will always reserve our right to take that decision at the time of it."
He outlined the UK's response to any killing of British citizens or military personnel in Iranian actions "would no doubt be proportionate".
British ships and helicopters are "on standby to assist" in the Middle East, where the UK is "changing the readiness" of its forces, Mr Wallace told the House of Commons.
The Defence Secretary also said "non-essential" British personnel have been relocated from Baghdad to the UK's base in Taji.
The UK is sending a team to Iraq to help the British military contingency plan for every eventuality, including for if personnel need to evacuate the country.
Around 400 British Army personnel are deployed in Iraq across three bases - Camp Taji near Baghdad, Union III in Baghdad, and Erbil in Iraqi Kurdistan.
The UK government is calling for calm over fears of retaliation.
Cover image: US Defence Secretary Mark Esper outside the White House (Picture: PA).