A rocket slammed into an Iraqi base where American troops are stationed in the remote province of Kirkuk, Iraqi and US security sources told AFP on Thursday night.
There were no immediate reports of casualties.
According to three separate Iraqi security sources, 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 base since December 27, when a volley of around 30 rockets killed a US contractor there and unleashed a dramatic escalation.
Washington blamed the rockets on Kataeb Hezbollah, a hardline Iraqi military faction close to Iran, and conducted retaliatory strikes that killed 25 of the group's fighters.