One person died after a rocket attack on Al Asad military base in western Iraq, which houses international forces, on Wednesday, the Pentagon said
The US civilian contractor suffered a cardiac arrest "while sheltering" from the attack, and despite being treated at the scene, died, Pentagon press secretary John Kirby said.
Similar attacks in the past were conducted by Iraqi militias aligned to Iran, but the US is yet to assign blame for Wednesday's incident.
"Iraqi security forces are on scene and investigating," Mr Kirby said.
"We cannot attribute responsibility at this time, and we do not have a complete picture of the extent of the damage. We stand by as needed to assist our Iraqi partners as they investigate."
Col Wayne Marotto, spokesman for the coalition fighting ISIS, said 10 rockets hit Al Asad base about 7.20am local time.
The rockets were launched eight kilometres from the base in Anbar province, a Baghdad Operations Command official told Reuters.
Later, the Iraqi military released a statement saying the attack did not cause significant losses and that security forces had found the launch site.
Iraqi Prime Minister Mustafa Al Kadhimi condemned the attacks.
"They are carried out by groups that have no true affiliation to Iraq, harming the progress the country has achieved," Mr Al Kadhimi said.
He said 60 per cent of coalition forces had left Iraq as a result of dialogue and not violence.
"We are proceeding with dialogue in accordance with Iraq's priorities to agree on a timetable for the departure of forces, and to agree on mechanisms that provide the training, support and advice of our security forces," Mr Al Kadhimi said.
Images on social media, which could not immediately be verified, showed Iraqi security forces at the scene of a burnt out truck with what appeared to be improvised rocket launchers attached to the roof.
The coalition's Joint Operations Command said Grad rockets were used. These are larger than those used in the February 15 attack on a US facility near Erbil International Airport, in northern Iraq, which killed a contractor.
The use of larger rockets suggests the group responsible had the intent to kill, unlike some attacks in the past that were more politically symbolic.
A leader in a Sunni tribal force in Baghdadi, a village not far from the base, said the rockets were fired from the Al Bayadir agricultural area.
Sabreen news website, thought to be linked to Iran-backed paramilitary group Asaib Ahl Al Haq, claimed that injured US personnel were moved from the site.
The website also said that Arash 4 rockets were fired – a version of the Grad made in Iran.
This is the second rocket attack in Iraq in just over a fortnight and comes two days before Pope Francis is due to visit the country.
Last week – in retaliation for the February 15 Erbil attack – the US struck Iran-aligned militia targets along the Iraq-Syria border.
This stoked fears of a possible repeat of last year's tit-for-tat attacks. These included the US air strike that killed Iranian Maj Gen Qassem Suleimani outside Baghdad International Airport.
The sprawling Al Asad facility has been targeted by Iran-backed groups on a number of occasions.
Iran also hit the base with a missile attack last year, in retaliation for the assassination of Suleimani.
This week, US Central Command released new footage of that attack, in which 11 ballistic missiles launched from Iran hit the base.
Dozens of soldiers suffered what the US military described as "traumatic brain injuries" that required treatment overseas. No US soldiers were killed in the attack, which caused heavy damage to the base.
Rocket salvos are a favoured method of attack by Iraqi militias linked to Iran. Most of these groups fall under the banner of the Popular Mobilisation Forces, a state-linked force formed in 2014 to fight ISIS.
British ambassador to Iraq Stephen Hickey condemned Wednesday's attack. A small number of British soldiers are stationed at the Al Asad base, training Iraqi security forces.
"Strongly condemn the rocket attacks on the global coalition base at Al Asad this morning. Coalition forces are in Iraq to fight Daesh at the invitation of the Iraqi government. These terrorist attacks undermine the fight against Daesh and destabilise Iraq," Mr Hickey said on Twitter.
There was no immediate response from the US government, beyond statements from its military.
This incident is likely to place more pressure on President Joe Biden's administration to get tough with Iran-backed groups.