BAGHDAD - A breakdown in trust between American forces in Iraq and the Iraqi Shia-dominated Popular Mobilisation forces is limiting the role that the US military can play in the war against Islamic State, Iraqi army and militia commanders have told Middle East Eye.

The tension within the uneasy alliance of US and Iran-backed militias fighting IS in Iraq has seen prominent Shia militias accuse US jets of targeting their fighters in the battlefield.

At the request of the Iraqi government, approximately 3,550 American troops - in addition to 1,500 multinational troops - have been approved for deployment to Iraq since last August. They have been sent to provide training, equipment, advice and assistance to the Iraqi security forces and have been deployed in al-Assad Air Base, Camp Taji, Besmaya base, Erbil and Habbinayah air base.

However, tensions have been growing.

Most of the Iraqi Shia militias, including Asaib Ahl al-Haq, Kataib Hezballah-Iraq and Badr Organisation, which represent the backbone of the Popular Mobilisation forces have been accusing the US military jets of deliberately targeting their fighters in the battlefield, instead of IS sites. The Popular Mobilisation force is a government-linked body established last June to co-ordinate the affairs of all the non-formal armed forces fighting IS.

The allegations are based on a number of airstrikes carried out by the US-led coalition that, by mistake, targeted several Iraqi military units and Popular Mobilisation forces that were stationed on the front lines in Salahudeen and Anbar province over the last six months, Iraqi security officials told MEE.

"We have a problem within the command and control system with the coalition forces and this has been causing a bug in coordination regarding the airstrikes," a senior Iraqi military officer, who declined to be named as he is not authorised to talk to the media, told MEE.

"These mistakes (strikes against the Iraqi forces and Shiite militias) were repeated several times in several areas. At the end of last year, more than 50 Shia fighters from the Popular Mobilisation forces were killed in an area near Balad (70 km north of Baghdad)," the officer said.

Most of the commanders and fighters of the armed Shia factions which are currently fighting the Sunni militants under the umbrella of the Popular Mobilisation had previously fought US troops during the period of 2003-2011 when the American army was regarded as the occupying force.

The three prominent Shia militias are linked to the great regional power of Iran which has armed, equipped and funded them for years. For these groups hostility to the US troops has always been an essential part of their ideology.

"We will never, ever participating in a military action (against IS militants) with the Americans. Not working or fighting alongside the US forces is one of our principles as an Islamic Resistance," Jawad al-Telebawi, the spokesperson of Asaib Ahl al-Haq, a key Shiite militia, told MEE.

"We cannot believe that despite the high quality of technology which is owned by the US-led coalition, their aircraft mistakenly and frequently struck our troops. It was obvious that they were interfering in favour of Daesh,” Telebawi said while using the Arabic word for IS.

A US-led military coalition spokesman denied targeting any anti-IS forces, and pointed out that the coordinates of strikes are determined by the Iraqi security forces and are subjected to detailed scrutiny by the coalition which works to ensure that there will be no civilian or allied casualties.

"The coalition has not struck any anti-IS forces locations. All strikes are coordinated and requested by the ISF [Iraqi security forces]. There is then a detailed process to ensure there is zero tolerance levels of civilian casualties, collateral damage, and that there has been positive identification of ISIL targets," the spokesperson told MEE in an email, while using another term for IS.

"The coalition conducted an investigation into an allegation that a coalition airstrike mistakenly killed Iraqi security forces near Beiji on December 24, 2014, and concluded that the claims could not be substantiated,” she added.

US troops' real role

Although the US-led international coalition recognises that the Popular Mobilisation forces play a major role in the destruction of IS's capabilities in Iraq, the level of cooperation between the two parties is almost non-existent, which reduces the effectiveness of airstrikes carried out by the coalition, Iraqi military officials said.

"They (US forces) refuse to participate in any battle whenever the forces of Hashid (the Popular Mobilisation) are involved, and their (the coalition) air raids near the battlefield end up being shy and ineffective," a federal security adviser linked to the Iraqi Ministry of Defence told MEE on the condition of anonymity.

"Now they (US forces) are totally focusing on the training of the Sunni tribesmen in Ain al-Assad and Habbaniyah air bases in Anbar," the adviser said.

Most of the Sunni-dominated areas in the northern and western parts of the country have been under the control of IS militants since last summer after the dramatic collapse of the Iraqi army.

The US administration believes that the participation of Sunnis in the liberation battles to retake their areas is crucial to defeat IS and break the link between the militant organisation and its local supporters.

So far the US-led coalition has trained five brigades consist of 9,000 troops including Iraqi security forces and Peshmerga (the Kurdish forces), the US-led coalition spokesman, said.

"The coalition forces are not boots on the ground. We are not involved in ground combat operations. We are involved in training Iraqi security forces, advising and assisting at the Iraqi headquarters locations and coordinating airstrikes with the Iraqi government," the spokesperson said.

"As counter-ISIL offensive operations continue, we can support all elements of anti-ISIL forces [including the] Iraqi Army and Counterterrorism Service, Popular Mobilisation Forces, Sunni tribal fighters, the Peshmerga, Federal Police and others," she added.