U.S. forces were attacked at an airbase west of Baghdad earlier today and a U.S. military aircraft responded in self-defence, U.S. officials said, in the first U.S. retaliation on Iraqi territory to dozens of recent militant drone and missile attacks.

Ain al-Asad airbase was attacked by a close-range ballistic missile which resulted in eight injuries and minor damage to infrastructure, two U.S. officials said.

The United States responded using an AC-130 aircraft that hit an Iranian-backed militia vehicle and a number of personnel involved in the attack.

There were several militant casualties, the officials said.

The U.S. had so far limited its response to numerous recent attacks against its forces in Iraq and neighbouring Syria, claimed by Iran-aligned Iraqi militia groups, to three separate sets of strikes in Syria.

The attacks began on Oct. 17 and have been linked by Iraqi militia groups to U.S. support for Israel in its bombardment of Gaza, following attacks by Gaza-based Palestinian militant group Hamas on Israel.

The attacks against U.S. targets have ended a year-long unilateral truce that Iraqi factions, some formed in the aftermath of the 2003 U.S. invasion to fight U.S. troops and others in 2014 to fight Islamic State, declared with Washington.

Tuesday's strike was the first reported by the U.S. in Iraq in more than two years.

Social media accounts linked to Iran-aligned Iraqi militias published a statement in the name of the "Islamic Resistance in Iraq", mourning a member who they said had been killed in battle against U.S. forces on Tuesday, without elaborating.

His killing is the first reported casualty in Iraq linked to the Gaza war, which has drawn in other factions in Iran's network of regional militias, known as the Axis of Resistance, such as Lebanon's Hezbollah.

U.S. and international forces that make up the global coalition to fight the remnants of Islamic State have been targeted more than 60 times in Iraq and Syria since Oct. 17, U.S. officials say.

Dozens of U.S. servicemen suffered minor injuries in the attacks but have all returned to duty, U.S. officials say.

The U.S. has 900 troops in Syria and 2,500 in Iraq on a mission it says aims to advise and assist local forces trying to prevent a resurgence of Islamic State, which in 2014 seized large swaths of both countries before being defeated.