Two rockets hit the Al-Balad air base, north of Baghdad, Iraqi security forces have said, the latest in a flurry of attacks on bases hosting US troops that has alarmed US officials.
It came as Washington considers deploying between 5,000 and 7,000 fresh troops to the Middle East to counter its arch-foe Iran, a US official told AFP late Thursday.
Thursday's attack with Katyusha rockets did not cause any casualties or material damage but "came close," a US official told AFP.
Washington has been concerned by a recent spate of attacks on Iraqi bases where some 5,200 US troops are deployed to help Iraqi forces ensure jihadists do not regroup.
The attacks, targeting either bases or the US embassy in Baghdad, have averaged more than one per week over the past six weeks.
"There is a spike in rocket attacks," a second US official said, adding that although they had caused no US casualties and little damage, they were increasingly worrying.
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Five rockets hit Al-Asad air base on December 3, just four days after Vice President Mike Pence visited troops there.
Security sources said they believed Kataib Hezbollah, a powerful Shiite faction close to Tehran and blacklisted by Washington, was responsible.
More than a dozen rockets hit the Qayyarah air base in northern Iraq last month, one of the largest attacks in recent months to hit an area where US troops are based.
There has been no claim of responsibility for the attacks and Washington has not blamed any particular faction.
But US Secretary of State Mike Pompeo has blamed similar attacks on Iran-aligned groups.
Iran holds vast sway in Iraq, especially among the more hardline elements of the Hashed al-Shaabi, a paramilitary force largely made up of Shiite militias backed by Tehran.
Asked whether the repeated rocket attacks made the Hashed a bigger threat to US troops than the Islamic State group, the official agreed.
"It is. The question is, when is someone going to call BS?" he said.
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 them and the Hashed 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.
"We all recognise the danger out here. Sometimes our Iraqi partners say, well what can I do?" the official said.
Tensions between Iran and the United States have soared since the Washington pulled out of a landmark nuclear agreement with Tehran last year and reimposed crippling sanctions.
Baghdad -- which is close to both countries and whose many security forces have been trained by either the US or Iran -- is worried about being caught in the middle.