A Marine carry team moves a transfer case containing the remains of Gunnery Sgt. Diego D. Pongo, on Wednesday. AP

Two American soldiers and a British soldier were killed Wednesday, a US official said, in the deadliest rocket attack in years on an Iraqi military base hosting foreign troops.

The attack threatens a dangerous escalation, with suspected US-led coalition air strikes promptly targeting Iran-aligned Iraqi fighters in neighbouring Syria, a war monitor said. At least 18 fighters were reported killed.

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On Wednesday evening, a volley of 18 Katyusha rockets hit the Taji air base north of Baghdad which hosts troops from the US-led coalition helping local forces battle jihadists.

A coalition statement said three of its personnel were killed and another 12 were wounded but it did not provide nationalities.

A US official, however, told AFP the dead included two Americans and a British national.

The Iraqi military said the rockets were fired from the back of a truck, but it did not comment on any casualties.

US Secretary of State Mike Pompeo and British Foreign Minister Dominic Raab condemned the attack in a telephone call.

The two "underscored that those responsible for the attacks must be held accountable," the US State Department said in a statement.

While there was no immediate claim of responsibility, Washington has blamed Iran-backed factions from Iraq's Hashed Al Shaabi military network, which is incorporated into the Iraqi state, for recent similar violence.