Iraqi pro-government tribesmen provided information that resulted in a US-led coalition air strike that killed 21 of their fighters, an Iraqi military command said on Thursday.

The coalition, which is bombing the Islamic State group in Iraq and Syria, announced that it was investigating the incident but did not confirm that it was responsible, while the Iraqi statement stated that the coalition carried out the strike.

"The air strike carried out by the international coalition... built on information provided by (tribal fighters) indicating the presence of hostile fire from one of the houses in the village of Kharaib Jabr," Iraq's Joint Operations Command said.

"The information was checked with its source, who confirmed the information, after which the air strike was carried out on the target selected based on the information," the JOC said.

The strike "martyred a number of (tribesmen) and wounded others, and an investigation was immediately opened," it added.

The air raid hit an area east of the town of Qayyarah, which was recaptured from IS in August, at about 1:00 am on Wednesday (2200 GMT Tuesday), according to a commander and a government minister whose tribe resides in the area.

The commander, Sheikh Nazhan Sakhr al-Lihaybi, said the tribesmen had succeeded in repelling an attack by IS in the area, and were bombed when they gathered after the fighting ended.

Qayyarah is located south of Mosul, Iraq's second city and the last in the country to be held by IS.

The US-led coalition announced on Wednesday that it was conducting a joint investigation with Iraqi forces into the incident, while a senior defence official told AFP that a coalition strike had likely killed about 20 pro-government fighters.

Iraqi forces are preparing for the battle to retake Mosul from the IS, an operation that will be supported by coalition air power.

The Mosul operation -- which Western officials have indicated could start this month -- will involve a heterogeneous coalition of sometimes rival Iraqi forces including soldiers, police, Kurdish peshmerga fighters, and both Sunni and Shiite militiamen.

After it is launched, these forces will have to fight their way through IS-held territory -- sometimes over distances of dozens of kilometres (miles) -- before surrounding the city and launching an assault to retake it.

The issue of which forces will actually enter the city is a contentious one, and there has been no public announcement of the roles the various forces will play.