Allies of the US including the UK could be at risk of retaliation from Iran following the killing of the country's most powerful military commander.
General Qassem Soleimani, the head of Iran's elite Quds Force, was killed along with up to six others in a drone attack ordered by US President Donald Trump near Baghdad's airport on Friday.
The US Defence Department said it killed Soleimani because he "was actively developing plans to attack American diplomats and service members in Iraq and throughout the region", sparking warnings of "harsh revenge" from Tehran.
American officials also accused Soleimani of approving the attacks on the US Embassy in Baghdad earlier this week.
The rapid rising of tensions led to to several experts warning of the risk of retaliations from Iran.
Ian Bond, foreign policy director at the Centre for European Reform, said on Twitter the airstrike was a "big escalation" by Mr Trump.
He added: "No doubt #Soleimani was very bad actor, with much blood on his hands.
"But killing non-state terrorists eg [al-Qaida founder Osama] bin Laden or [so-called Islamic State leader Abu Bakr al-] Baghdadi very different from killing senior official of internationally-recognised state.
"Big escalation by Trump, and a lawless step that increases risk to US and allies."
Dr Jack Watling, a research fellow at the Royal United Services Institute, said the attack was "very significant" because it was a "declared assassination" outside a declared armed conflict.
However he believed Iran was not likely to want to provoke a war with the US.
Dr Watling said: "The significance of this strike is that it is a declared assassination of a senior officer in another state with whom the US is not in a declared armed conflict and conducted on the territory of a third party.
"That's a very significant development in and of itself.
"Ultimately Iran does not want to provoke a full-scale conflict.
"I would expect there will be attacks on US forces, but they will be conducted with care."
Dr Watling added if the UK was seen by Iran to be participating in US actions it could capture or arrest British citizens in the region.
He added: "The Iranians do not draw a direct line between the UK and US, however, if the UK is perceived to be participating in US actions then they will directly target UK interests.
"The UK is not automatically the first target. Citizens in Iraq, Syria and Lebanon would certainly potentially be at risk, dual-nationalities in Iran will certainly be at risk of arrest under espionage charges."
British Foreign Secretary Dominic Raab urged all parties to de-escalate the situation, saying further conflict is "in none of our interests".
Additional reporting by PA Media
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