Veterans of the US occupation of Iraq have been joining Kurdish units fighting the Islamic State of Iraq and the Levant (ISIL) group in northern Iraq.

Al Jazeera spoke to a number of the men, who had crossed into Iraq a few months ago, and had since taken part in clashes against ISIL fighters, alongside Kurdish Peshmerga troops.

One former member of the US Marine Corps, who went by the name Kurt, said the volunteers have been moved to take part in the conflict by reports of atrocities ISIL had committed since they seized large parts of Iraq a year ago.

"I just thought it was the right thing to do. ... I saw a lot of the atrocities happening via the news, the beheading and the slave trade," he said.

Kurt explained that he and other former US soldiers had been smuggled across Turkey's border by a group called Frame, which had since disbanded.

Al Jazeera's Zeina Khodr, reporting from southern Kirkuk, said the men brought military experience with them but were short on the equipment needed to fend off ISIL fighters.

"The arms they carry are good for urban warfare, not for a battlefield like this, where there's open terrain," she said.

Another volunteer she spoke to said his weapons could not hit targets beyond 200m.

The fighters have been welcomed by Kurdish authorities and factions, which have made use of Western volunteers in both Iraq and Syria.

READ MORE: Kurds and ISIL battle over Kirkuk

However, the Iraqi government in Baghdad has rejected their presence.

The US military has reinforced Kurdish and Iraqi government campaigns to retake territory held by ISIL through an aerial campaign, and by sending non-combat military advisers.

Despite the presence of Western air power, ISIL has continued to make gains, most recently taking Anbar's provincial capital, Ramadi, after more than a year of clashes in the city.

The Pentagon has ruled out sending combat troops to fight ISIL.