Two Americans and one British were killed and about a dozen people were wounded when 18 small rockets hit Iraq's Taji military camp north of Baghdad on Wednesday, according to Reuters's sources.
The officials, who cautioned the death toll may rise given the severity of some of the injuries, said it was too soon to assign blame. Any indication that Iran-backed militia was responsible could spark a new round of confrontation between the United States and Iran.
The last military escalation led to a U.S. strike in January that killed top Iranian General Qassem Soleimani, which, in turn, led Iran to fire missiles at a base in Iraq hosting U.S. forces, leaving more than 100 troops with brain injuries.
U.S. Secretary of State Mike Pompeo has argued the killing of Soleimani was necessary for part to deter Iran and Iran-backed groups from additional attacks.
The U.S.-led military coalition in Iraq said in a statement that 18 107 mm Katyusha rockets struck the base and suggested they may have been fired from a truck.
It confirmed three personnel were killed and about a dozen wounded but declined to disclose the nationalities of the casualties or offer any other identifying information.
"The attack is under investigation by the Coalition and Iraqi Security Forces," the coalition said.
Earlier on Wednesday, the Iraqi military said no casualties had been reported in the attack.
In a sign of concern that tensions between the United States and Iran could be headed toward open conflict, the Democratic-led U.S. House of Representatives passed legislation on Wednesday to limit President Donald Trump's ability to wage war against Iran.
U.S. President Donald Trump at a news briefing inside the Cabinet Room at the White House in Washington, U.S., March 11, 2020. /Reuters
The Republican president has been engaged in a maximum-pressure campaign of renewed sanctions and near-constant rhetorical conflict against Iran, after pulling the United States out of the international nuclear deal reached during the administration of his Democratic predecessor, Barack Obama.
Tension between Washington and Tehran has mostly played out on Iraqi soil in recent months.
Iran-backed paramilitary groups have regularly been rocketing and shelling bases in Iraq that host U.S. forces and the area around the U.S. Embassy in Baghdad.
The Kataib Hezbollah militia was blamed for an attack on a base last year that killed an American contractor, which led to retaliatory U.S. strikes against it in Iraq and Syria.
Syrian state media reported on Wednesday that unidentified jets hit targets southeast of a Syrian town along the border with Iraq.
But there was no indication so far that those strikes were by the United States. U.S. officials told Reuters the United States had not carried out any retaliatory strikes at this point.