Iran has struck back at the United States for killing a top Revolutionary Guard commander, firing ballistic missiles at two military bases in Iraq containing British and American troops.
The Ministry of Defence has confirmed there have been no UK or US casualties in the missile strikes.
The Iraqi government said there were no casualties among Iraqi forces.
Iran's Revolutionary Guard has taken responsibility for the attacks, in retaliation for General Qassem Soleimani's death, according to Iranian state TV.
A total of 15 missiles were fired, two US officials said, with ten hitting Al Asad air base and one striking a base in Erbil.
Four of the missiles failed, said the officials.
The White House says President Donald Trump has been briefed and is "monitoring the situation closely".
He has previously threatened "disproportionate" action if Iran targeted Americans or US assets.
Following the attack, Mr Trump tweeted that "all is well" and said he would be making a statement in due course.
The UK's Foreign Secretary Dominic Raab has added to calls from Europe for de-escalation in the region following the military base attacks.
"We condemn this attack on Iraqi military bases hosting coalition - including British - forces", Mr Raab said.
"We are concerned by reports of casualties and use of ballistic missiles.
"We urge Iran not to repeat these reckless and dangerous attacks, and instead to pursue urgent de-escalation.
"A war in the Middle East would only benefit Daesh and other terrorist groups."
The US earlier this week denied that it was planning to withdraw troops from Iraq, in light of tensions with Iran, with American officials said they were anticipating Iran to respond in the form of a "major" attack.
More than 5,000 US troops are deployed in Iraq, while the UK has around 400 on the ground in the country in a training capacity.
Iran's Foreign Minister Javad Zarif has said the country "took & concluded proportionate measures in self-defense under Article 51 of UN Charter targeting base from which cowardly armed attack against our citizens & senior officials were launched".
"We do not seek escalation or war, but will defend ourselves against any aggression," he added.
Chapter VII: Article 51 of the UN Charter details the "inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security".
The US-led coalition's training mission of Iraqi forces in the country had already been suspended prior to the attacks on the two military bases, including the UK's role.
Speaking in Brussels yesterday, NATO Secretary General Jens Stoltenberg also took the opportunity to urge against further Iranian aggression.
''In everything that we do, the safety of our personnel is paramount,'' Mr Stoltenberg said after a meeting of the North Atlantic Council.
''As such, we have for the time being suspended our training on the ground, and we are taking all precautions necessary to protect our people.
"We are keeping the situation under close review.''
Last month, the US military carried out "precision defensive strikes" against Iranian-backed paramilitaries in Iraq and Syria.
Five facilities controlled by the Kata'ib Hezbollah group were successfully targeted, in response to repeated attacks on Iraqi military bases hosting US forces.
Resulting escalations have led to the killing of General Soleimani - pushing Tehran to abandon the remaining limits to its 2015 nuclear deal.
''We agree Iran must never acquire a nuclear weapon; we share concern about Iran's missile tests, and we are united in condemning Iran's support for a variety of different terrorist groups," Mr Stoltenberg said on Tuesday.
Cover image: Coalition forces in Al Asad Air Base, one of the sites targeted by Iran, pictured in 2019 (Picture: US Department of Defense).