US President Donald Trump has said a top Iranian general killed in a US air strike "should have been taken out many years ago".
General Qassem Soleimani, who was killed in a drone attack at Baghdad airport on Friday morning, was responsible for Iran's military campaigns abroad and allegedly orchestrated a number of attacks on US military personnel and bases.
Mr Trump echoed the stance of the Pentagon, saying Gen Soleimani was "plotting to kill" many Americans and that he was "directly and indirectly responsible for the death of millions of people".
UK Foreign Secretary Dominic Raab urged "all parties to de-escalate", while the country's president, Hassan Rouhani, called the killing a "heinous crime" and vowed his country would "take revenge".
Thousands of worshippers in the Iranian capital Tehran took to the streets after Friday prayers to condemn the killing, chanting "death to deceitful America".
American and British flags were also seen being burnt by demonstrators.
Washington has told American citizens to leave Iraq "immediately" and has shut its embassy in Baghdad, days after it was attacked by Iran-backed militia and their supporters.
Experts have voiced concern that the attack could lead to even greater tensions between Iran and the West.
Ian Bond, foreign policy director at the think-tank Centre for European Reform, said on Twitter: "Big escalation by Trump, and a lawless step that increases risk to US and allies."
Dr Jack Watling, research fellow at the Royal United Services Institute, warned that if Iran believes the UK is "participating in US actions then they will directly target UK interests".
Meanwhile, the UK is reviewing its "force protection" in the region.
Around 400 British Army personnel are deployed in Iraq, in a non-combat role, across three bases.
The UK also has four Royal Navy ships, including a Type 45 destroyer and Type 23 frigate, operating in the Gulf.
Relations between Iran and the West have detoriated rapidly since the US left the 2015 nuclear deal, sparking tensions in the Strait of Hormuz and rows over sanctions.
Cover Image: President Donald Trump (Picture: PA).