Captain America shut down social media speculation that he was apparently signing an Israeli bomb for use in Gaza -- taking to an IG Story Thursday to clarify those claims were BS ... adding the photo was actually taken during a USO tour back in Dec 2016.

Chris says he participated in the trip alongside a group of actors, athletes and musicians to express gratitude to U.S. service members ...  and that what he signed was commemorative.

He explained, "The object I was asked to sign is not a bomb, or a missile, or a weapon of any kind. It's an inert object used for training or display purposes only. You can read the quote from the Air Force in the next story."

The accompanying Air Force statement read ... "The object Chris Evans is signing in the USO tour photo from 2016 is an explosive ordnance disposal (EOD) inert training aid. The object is meant to model an artillery shell and is for display and training purposes only."

Chris' picture was actually snapped at the Incirlik Air Base in Turkey, where he and his crew, including his "Avengers" co-star Scarlett Johansson, spread holiday cheer to deployed U.S. troops in a trip organized by the United Service Organizations.

The picture gained traction online just after Nikki Haley's Memorial Day visit to Israel, where she wrote "Finish Them" on Israeli missiles, sparking considerable controversy and likely fueling the spread of misinformation online.

But the pic had been circulating for some time, and Jennifer Passey, director of communications for the USO, confirmed to AFP in Feb it was indeed taken during their 2016 USO Holiday Tour. While Chris's clarification is appreciated, some are still mad.

They're saying he should speak out on the situation, or offer a donation to Gaza ... he hasn't done that yet though -- and he hasn't spoken about the Middle-East conflict in general.