An ISIL propaganda video showing the deadly ambush of American troops in Niger raised questions on Monday as to the nature of the mission and why the soldiers had been left so vulnerable.

The October 4 attack occurred as a unit of 12 American special forces soldiers and 30 Nigerien troops returned from the village of Tongo Tongo, near the border with Mali. They were targeted by a group of some 50 ISIL-affiliated fighters equipped with small arms, grenades, and trucks mounted with guns.

At least five Nigerien troops were killed along with four US soldiers in the attack.

The video, distributed by a pro-ISIL news agency, includes graphic footage taken by a soldier wearing a helmet camera. It shows the chaos of the attack, including the soldier wearing the camera being shot dead, with fighters stalking past his body.

The Pentagon "is aware of alleged photos and IS [Islamic State of Iraq and the Levant] propaganda video from the Oct 4, 2017 terrorist attack in Niger. The release of these materials demonstrates the depravity of the enemy we are fighting," it said in a statement.

Ill-equipped

The nine-minute video includes an image of ISIL leader Abu Bakr al-Baghdadi and footage of pick-up trucks rolling through a desert landscape. One of the vehicles is flying an ISIL flag and other trucks are packed with what appear to be fighters.

The mission was supposed to be a low-risk patrol and the soldiers were clearly ill-equipped for the scale of the attack.

Footage shows the US troops wearing only light body armour, desperately seeking cover behind an unarmored SUV while coming under heavy fire.

In a frantic bid to find some sort of concealment, the troops deployed red smoke grenades but the parched landscape of scrub and dirt provided no effective cover.

At one point in the video, a US soldier is shot and a comrade attempts to pull him to cover behind the SUV. As their position is overrun, they have no choice but to try to run away, but there is nowhere to hide.

"Knowing that they were asked to try and complete and execute this type of mission with that type of equipment, I just could not believe it," Republican Congressman Marc Veasey told CBS news.

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Questions remain about what intelligence failures may have occurred that allowed such a large attack, and why the soldiers did not get immediate backup or air support.

The Americans had been on a joint patrol with Nigerien counterparts they were training when they were ambushed by motorcycle-riding and car-driving gunmen in the Tillaberi region in the Niger's southwest.

Pentagon investigators have completed a probe into the incident, and that report is currently under review by defence chief James Mattis.