The counter-terrorism division of Iraq's autonomous Kurdish region on Wednesday accused the Hashed al-Shaabi, a state-sponsored paramilitary network dominated by pro-Iran forces, of targeting US forces in a rocket attack.
"Six rockets were fired from Nineveh province by the Hashed al-Shaabi, targeting Arbil airport," where US troops are based, on Wednesday, it said in a statement.
Souran Ali, a commander of the Democratic Party of Iranian Kurdistan (PDK-I) -- an Iranian opposition group that operates in Iraq's Kurdish region -- earlier said three rockets had been fired, without causing casualties.
Three rockets "targeted at 8.30pm (1730 GMT) a headquarters of the PDK-I two kilometres" from Arbil airport, he said.
Video footage circulating online purported to show fires started by the rockets.
Hoshyar Zebari, a former Iraqi foreign minister and an Iraqi Kurdish politician, said on Twitter that the rockets were fired by the "same groups who are attacking #USEmbassy in #Baghdad and its convoys. Action is needed to stop it."
Around 40 rocket attacks have targeted US interests in Iraq since early August, including one on Monday that hit a village close to Baghdad airport, where American troops are stationed. That attack killed five children and two women from the same family.
The US State Department on Tuesday pointed the finger of blame at Iran-backed militias operating in Iraq, saying they "pose an unacceptable danger" in Iraq.
Washington has threatened to close its embassy in Iraq and withdraw its 3,000 troops from the country unless the rockets stop.