Grainy photographs posted and circulated on Twitter purportedly show a soldier in US uniform lying on the dry-earthen ground, a US flag patch on his chest.

The image looks like it was recorded from another soldier's helmet camera. The exact provenance of the photos is murky, but the original Twitter account to post the pictures said they had been collected by an Islamic State affiliate.

"I'm aware of the report, and we have not confirmed the authenticity of that," Pentagon spokeswoman Dana White said Thursday.

One tweet, now deleted, claimed there was an associated video.

The US military's Africa command said it was aware of the Twitter postings purporting to show the October 4 ambush in Tongo Tongo, Niger.

"We are reviewing the post and determining the veracity of the tweet and the assertions that there is an associated video," AFRICOM said in a statement.

The ambush occurred as a unit of 12 American special forces soldiers and 30 Nigerien troops returned from the village near the border with Mali.

They were attacked by a group of some 50 fighters affiliated with IS and equipped with small arms, grenades and trucks mounted with guns.

Four American soldiers were killed along with at least five Nigerien troops. The body of one US soldier, Sergeant La David Johnson, was not recovered until the following day.

Questions remain about what intelligence failures may have occurred that allowed such a large attack, but the Pentagon has remained tight-lipped about the circumstances surrounding the ambush as well as the nature of the mission while it conducts a probe.

US troops frequently wear cameras while out on missions, sometimes even using their own GoPro-style devices.