The photos show an American portable rocket launcher, radio, grenades and other gear not commonly used by Afghan troops, as well as close up views of identification cards for a US Army soldier, Specialist Ryan Larson.
The US military command in Kabul denied any suggestion the soldier had been captured, saying he "has been accounted for and remains in a duty status within his unit."
"SPC Larson was attached to a unit conducting a partnered (operation) with Afghan Forces," US military spokesman Commander Ron Flesvig said in an emailed statement Sunday. "The soldier's I.D. and some of the equipment were left behind after the (operation). The loss of personal identification is unfortunate."
In July, US commanders said at least five special forces were injured in fighting in the province, according to Reuters.
The website that published the photos speculated that the equipment and weapons were left behind during that engagement, but Flesvig said American officials are still trying to determine exactly when and how it was lost.
The push in Nangarhar came after US President Barack Obama cleared American troops to take a more active role in fighting terrorists in Afghanistan.
Besides advising work and special operations missions, American aircraft deployed at least 545 weapons in the first six months of 2016.