"Multiple ballistic missiles and rockets" were fired by Iranian-backed militants at Al-Assad Airbase in western Iraq late Saturday, the US military said, leading to one Iraqi and possible American casualties.

"It was a very serious attack, using a capability of ballistic missiles that posed a genuine threat," White House Deputy National Security Advisor Jon Finer said Sunday.

"We are going to respond... to establish deterrence in these situations, and to hold these groups accountable that continue to attack us," Finer added during his appearance on ABC's This Week.

"You can be assured that we are taking this extremely seriously."

Most of the projectiles fired at the base were intercepted by air defense systems, Finer and the Pentagon said.

Since mid-October, there have been dozens of attacks on the approximately 2,500 US troops in Iraq and the around 900 in Syria, deployed there with other coalition forces to fight jihadists of the Islamic State group.

Most incidents, including Saturday's attack, have been claimed by "Islamic Resistance in Iraq," a loose alliance of Iran-linked armed groups that oppose US support for Israel in the Gaza conflict.

The use of ballistic missiles marks an escalation in the attacks, which had previously been carried out with lower-tech rockets and drones.

Saturday's air base attack comes amid soaring tensions in the Middle East following the outbreak of the Israel-Hamas war on October 7.