A rocket attack has slammed into an Iraqi base in the remote province of Kirkuk where US troops are stationed, security sources said.
There were no immediate reports of casualties, AFP news agency citing Iraqi and US security sources reported on Thursday.
Three separate Iraqi security sources told AFP that the Katyusha rocket hit the K1 base at around 8:45pm local time (1745 GMT) and US military aircraft immediately began flying low over the area.
It was the first attack on the K1 base since December 27, when a volley of about 30 rockets killed a US contractor there, which Washington blamed on Kataib Hezbollah, an Iraqi military faction close to Iran.
The US then carried out retaliatory attacks that killed 25 Kataib Hezbollah fighters.
Days later, another strike killed the head of Iran's elite Islamic Revolutionary Guard Corps (IRGC) Quds Force, General Qassem Soleimani and Kataib Hezbollah cofounder, Abu Mahdi al-Muhandis.
Reporting from Baghdad, Al Jazeera's Simona Foltyn said the attack came at the end of a 40-day mourning in Iran over Soleimani's death.
"There is the possibility that this has to do with the end of the 40-day mourning period for Qassem Soleimani today," she said, adding that the attack could reignite tensions in the region.
The attack infuriated Shia Iraqi legislators who voted to remove more than 5,000 US troops deployed in the country in a January 3 Parliament session.
Iran retaliated for Soleimani's killing with a barrage of missiles that targeted two airbases hosting US troops in Irbil and Ain al-Asad. The troops had prior warning and none were killed, but more than 100 have since been diagnosed with traumatic brain injuries.
Iran and the US have since refrained from further escalation, but the issue of US troops has monopolised Iraqi politics.
"There is a fear that such rocket attacks will trigger a response that will lead once again into an escalation in a crisis that has only recently calmed down," Foltyn said.