A group calling itself Saraya Awliya Al Dam has reportedly claimed responsibility for Monday night's attack on a US military base near Erbil, in the Kurdish Region of Iraq.
The attack killed a civilian contractor and injured six people, including a US serviceman.
While the claim could not immediately be verified, the past 18 months have seen the emergence of a number of new Shiite militia groups in Iraq.
These have been characterised as front organisations for official Iran-backed groups within a larger Iraqi militia organisation – the Popular Mobilisation Forces (PMF).
Kurdish intelligence services later found Iranian Fajr-1 rockets still intact at their launch site, within the Kurdish region of Iraq.
The Fajr 1 has a short range of around eight kilometres, confirming the view of security sources that the projectiles used in the Erbil attack had been launched by militia fighters within the semi-autonomous region.
The PMF is linked to the Iraqi government, but a number of hardline elements and the leadership of the organisation often follow Iranian orders.
It has sent units to fight in Syria and has attacked international coalition forces in Iraq.
A previous attack on the base near Erbil originated in the Nineveh plains, outside the northern Iraqi city of Mosul, an area under the control of Iran-backed militias.
In the hours before the Erbil attack, another group calling itself Ashab Ahl Kahf fired rockets at Turkish forces in northern Iraq, who were undertaking an offensive against the Kurdistan Workers' Party militia.
Ashab Ahl Kahf is thought to be linked to Asaib Ahl Al Haq, a hardline PMF militia with a political wing, Sadiqun, which has 15 seats in Iraq's parliament.
US Secretary of State Antony Blinken said he was "outraged" by the attack in Erbil, and pledged that the US would work with Iraqi authorities in investigations to hold those responsible to account.
However, this could be a difficult task. Even if the attacks can be traced to Iran-backed groups, hardline pro-Iranian militias enjoy a degree of protection from their allied political parties in Iraq's parliament.
Calling for an investigation is in stark contrast to what occurred following a similar incident under the administration of former president Donald Trump.
When rockets killed a US civilian contractor on December 27, 2019, the situation escalated into US airstrikes and PMF retaliation.
This culminated in the assassination of Iran's Maj Gen Qassem Suleimani, commander of the Islamic Revolutionary Guard Corps' Quds Force, and in response, an Iranian ballistic missile attack on US bases in Iraq.
War was averted when the US decided further escalation could lead to full-scale conflict.