At least seven rockets hit an Iraqi military airbase which houses US contractors in Balad, north of Baghdad, on Monday.
There were no reported casualties, Iraqi security officials said.
The Iraqi military said two additional rockets fell outside the Balad airbase, 80 kilometres from the capital, and caused damage to a house.
No group immediately claimed responsibility for the attack, but armed groups that some Iraqi officials say are backed by Iran have claimed similar incidents in the past.
The attack was reported at 11pm local time by Sabreen news, an online news service sympathetic to Iran-backed Iraqi militias.
Iraqi security forces later tweeted on an official account images of a nearby house that had been damaged by one of the rockets.
US contractors
Balad airbase houses contractors with US defence company Sallyport Global.
They are assisting the Iraqi Air Force on a long-term contract to provide maintenance for Iraq's fleet of 36 F-16 fighter jets.
But this has not stopped Iran-backed militias – who are ostensibly linked to the Iraqi government – from attacking the site.
The militias backed by Iran are part of the Popular Mobilisation Forces (PMF) an umbrella organisation of groups. Some were created to fight ISIS, while others were formed in Iran in the 1980s and went on to attack US and British forces after the 2003 invasion of Iraq.
Some PMF groups are linked to Iraqi political parties and enjoy a degree of protection in Iraq; all receive government salaries.
But the political controversy this has attracted for Iran-affiliated groups has led to the formation of new Shiite militias who attack coalition forces.
Many analysts, including Michael Knights who was interviewed by The National in January, believe new groups, such as Ashab Ahl Kahf, are simply fronts for official Iran-backed PMF groups.
As a result of sporadic attacks at Balad, US contractors have been evacuated on several occasions during heightened tensions.
This included following the assassination of Iran's Maj Gen Qassem Suleimani, commander of the Islamic Revolutionary Guard Corps' Quds Force, in a US drone strike at Baghdad International Airport in January 2020.
Attacks on Balad could make it more difficult for the US and Iraq to keep the aircraft serviceable – a significant blow to Iraq's nascent airpower.
Iraq's F-16s have cost the government at least $4 billion, including maintenance costs, ground crew and pilot training.
Speaking of the growing contest between the US and Iran in Iraq, an unnamed Iraqi official told The National in January: "Unfortunately, the leadership in Baghdad has given the all-clear for Tehran and Washington to do as they please in the country."
Erbil attack
The latest Balad attack is reminiscent of another rocket strike on a US military facility, next to Erbil International Airport, in the Kurdistan Region of Iraq, on February 15.
A contractor working with US forces was killed in the attack and eight contractors and one US service member were injured.
US officials blame Iran-backed militias for regular rocket attacks on US facilities in Iraq, including near the embassy in Baghdad.
Iran-aligned paramilitary groups have demanded that all foreign troops – including US forces who number around 2,500 in Iraq – leave the country, calling their presence an "occupation".
A US-led coalition, whose mission is to fight ISIS militants, is still stationed in Iraq, as is a Nato-led mission that trains Iraqi security forces.
