Iran launched 11 missiles at a US air base at Ain al-Assad and another at a base in Erbil on January 8 in retaliation for the killing days earlier of Iranian General Qassem Soleimani in a US drone strike in Baghdad.

No US troops were killed but dozens suffered traumatic brain injuries from the explosions, and Washington wants to deploy Patriot missiles to better protect the bases, which house some of the 5,200 US military personnel deployed in Iraq.

The Patriot systems are composed of high performance radars and interceptor missiles capable of destroying incoming ballistic missiles in flight.

Questioned Thursday about the delay in deploying the system, Esper told reporters the Iraqi government, which apparently is divided over the US military presence in the country, has yet to give it the go-ahead.

"We need the permission of the Iraqis," he said. "That's one issue. There may be others with regard to placement and things like that."

General Mark Milley, the chairman of the Joint Chiefs of Staff, noted that a Patriot battalion is a relatively large organization, and the mechanics of deploying one to Iraq "will have to be worked out. And that is, in fact, ongoing."

Iraq denounced Soleimani's killing as an assault on its sovereignty, and charged that the international coalition in Iraq had overstepped its mandate.

The US-led coalition was formed in 2014 to fight the Islamic State group, which at the time had seized control of a third of Iraq's territory and large swaths of Syria. The coalition includes troops from 76 countries.

On January 5, the Iraqi parliament voted in favor of the withdrawal of US forces from the country. Coalition operations have been suspended since then.