The assault on the Ain Al-Assad base came as pro-Tehran groups hailed the election of Iranian ultraconservative cleric Ebrahim Raisi as the new president of the Islamic republic, with some saying it reflected the "failure" of America's "pawns" in the region.

The attack also comes 10 days after the United States offered a reward of up to $3 million for information on strikes against its citizens in Iraq.

US interests in Iraq have come under repeated attack since October 2019, including with rockets, with Washington routinely blaming them on Iran-backed factions.

Since the beginning of the year, a total of 43 attacks have targeted the US embassy in Baghdad, Iraqi bases housing US troops or Iraqi convoys carrying logistical support.

Pro-Iran groups in Iraq are campaigning for the departure of the remaining 2,500 US troops deployed in the country as part of an international coalition to fight the Islamic State group. The Iraqi government declared that fight won in late 2017.

On June 10 the US Department of State's Rewards for Justice programme urged Iraqis with any information concerning anti-American attacks to come forward.

"Faithful people of Iraq, cowardly terrorists are attacking US diplomatic missions in Iraq, then they are fleeing to hide among civilians," it said in a statement in Arabic on Twitter.

"America is offering a reward of up to $3 million for information on planned attacks or past ones against American diplomatic installations," it added.

A telephone number was provided for anyone wishing to call in information, or share it via the messaging apps Whatsapp, Telegram or Signal.

The offer of the reward was made a day after an attack against Baghdad airport -- which also houses US troops -- using three explosive packed drones.

Experts say the use of such drones marks an escalation in attacks against American interests by pro-Iranian forces.

Iraqi authorities have repeatedly blamed "outlaws" for carrying out "terrorist" attacks with rockets or explosive-laded drones, but have struggled to identify those behind the assaults.