A rocket attack in northern Iraq killed an American contractor and wounded several military personnel on Friday, the first US casualties from a string of recent strikes, the international coalition against the Islamic State group said.
While the coalition did not attribute blame for the incident, it threatens to escalate already-high tensions between Washington and Tehran, which backs various paramilitary groups in Iraq that the United States has accused of being behind rocket attacks on its interests.
"One US civilian contractor was killed and several US service members and Iraqi personnel were wounded in a rocket attack on an Iraqi military base in Kirkuk," the US-led coalition said in a statement.
Federal security forces and Shia militia units - as well as Islamic State (IS) group sleeper cells - all have a presence in volatile Kirkuk province, which is claimed by both Iraq's Kurdish minority and Arab majority.
"Iraqi Security Forces are leading the response and investigation" into the attack, which took place at 7:20 pm (2220 GMT), the coalition said.
A US official with knowledge of the investigation told AFP on condition of anonymity that at least 30 rockets hit the base, including an ammunition depot, causing more explosions, while four more rockets were found in their tubes in a truck at the launch point.
The official described the attack as the biggest in the series of rocket strikes launched against US interests in the country since late October, killing one Iraqi soldier and leaving others wounded, as well as causing material damage in the vicinity of the US embassy in Baghdad's Green Zone.
A US source has said that pro-Iran factions in Iraq are now considered a more significant threat to American soldiers than the IS - the threat that saw Washington deploy thousands of troops to the country to assist Baghdad in countering the jihadists' sweeping 2014 offensive.
Five rockets hit Al-Asad air base on December 3, just four days after US Vice President Mike Pence visited troops there.
More than a dozen rockets hit the Qayyarah air base in northern Iraq in November, one of the largest attacks in recent months to hit an area where US troops are based.
Multiple US diplomatic and military sources have told AFP of their growing frustration with such attacks.
They say they are relying on their Iraqi partners to play a "de-conflicting" role between American forces and the Hashed al-Shaabi - an umbrella organisation for paramilitary groups that is largely made up of Iran-backed Shia militias - to prevent any clashes.
That is a complicated task, as the Hashed has been ordered to integrate with the regular security forces, but many of its fighters continue to operate with some independence.
US Defence Secretary Mark Esper told reporters earlier this month that he had expressed "concern about the optics in attacks on bases in Iraq where US troops and material might be," in a call with outgoing prime minister Adel Abdel Mahdi.
The US has "a right of self-defence, that we would ask our Iraqi partners to take proactive actions... to get that under control, because it's not good for anybody," he said.
Abdel Mahdi's office called on everyone "to spare no effort to prevent an escalation that will threaten all parties," warning that "unilateral decisions will trigger negative reactions that will make it more difficult to control the situation."
Tensions between Iran and the United States have soared since Washington pulled out of a landmark nuclear agreement with Tehran last year and enacted crippling sanctions.
Baghdad - which is close to both countries - is worried about being caught in the middle.
Meanwhile, thousands have taken to the streets in Iraq's capital and across the south in recent weeks to protest against Iran's kingmaking influence, as Baghdad continues its search for the next prime minister.
Anti-government rallies have rocked Baghdad and the Shia-majority south since October 1, with demonstrators calling for a complete overhaul of a regime they deem corrupt, inefficient and overly beholden to Tehran.
In a bid to secure the necessary parliamentary majority for a new premier, Shia powerhouse Iran enlisted the services of a Lebanese Hezbollah official to negotiate with Sunni and Kurdish parties.
The post of prime minister is by convention held by a Shia in Iraq's post-2003 political system.
Around 460 people have been killed since protests erupted in October, and some 25,000 have been wounded.
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