BAGHDAD - 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.  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, said 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.

Within hours of Wednesday’s attack, three warplanes likely belonging to the US-led coalition bombed Hashed factions stationed on the Syrian side of the border with Iraq, according to the Syrian Observatory for Human Rights war monitor.

At least 18 Iraqi fighters were killed as “ten explosions shook the area near Albukamal,” a Syrian border town with a heavy Hashed presence, said Observatory head Rami Abdel Rahman. Hardline Hashed factions have fought alongside Syrian government forces for several years and have been targeted by both coalition and Israeli air strikes in Syria.