Iraq has said the US-led international coalition is responsible for an “unprovoked” attack on an Iraqi security force today, according to the prime minister’s office.

“The attack is a dangerous escalation and a violation of Iraq’s sovereignty,” the prime minister’s office said.

At least four paramilitary fighters were killed and six others injured today in a drone attack on the headquarters of an Iranian-backed paramilitary group in eastern Baghdad, police and security sources told Reuters.

Police sources and eyewitnesses said at least two rockets hit a building used by the Iraqi paramilitary group al-Nujaba’a.

A representative of the organization stated that the report increased to four dead after one fighter succumbed to his wounds. Health sources confirmed the tally.

A local al-Nujaba’a commander and one of his associates were killed in the attack, police and the group’s spokesman said.

The spokesman and two Iraqi paramilitary commanders accused the United States of carrying out the attack and threatened retaliation.

“We will fight back and make the Americans to regret making this attack”said Abu Agil al-Moussawi, a local Iraqi paramilitary commander.

There is currently no comment from Washington.

Iraqi police and security sources said that, pending a government investigation, they had no further information on who might have carried out the strike.

Police and paramilitary sources said rockets targeted a vehicle inside Nujaba’s headquarters, killing a local commander of the group and an associate. Two other members of the group were killed in the attack.

Video images released by bystanders to paramilitary websites show a damaged vehicle on fire that was allegedly targeted during the attack.

Reuters cannot independently confirm the images.

Iraqi security forces were deployed to the area and a security team arrived at the scene for initial investigations, two security sources said.

Last month the United States launched retaliatory airstrikes in Iraq after a drone attack by Iran-linked militants seriously wounded one member of the US forces and injured two others.

US troops have been attacked at least 100 times in Iraq and Syria since the Israel-Hamas war began in October, usually by rockets and kamikaze drones.

The United States has 900 troops in Syria and 2,500 in Iraq in a mission it says is intended to advise and assist local forces trying to prevent a resurgence of Islamic State, which in 2014 seized large parts of the two countries before being defeated.

Nujaba-a is one of the factions belonging to Hasd al-Shaabi, a coalition of Shiite former paramilitaries close to Iran that has now been integrated into regular Iraqi forces.

These forces have been the target of bombings in Iraq in recent weeks, many of which have been claimed by the United States.