Andrew Shumack, Jr, 25, believed to be from Pennsylvania, passed through St Petersburg from Western Europe on his way to Chechnya, making contact with local journalists and media outlets here.
But journalists in Grozny who found his backpack, sleeping bag and documents (including a copy of his passport) abandoned in his hotel there, fear for his safety.
Mr Shumack left his heavily guarded hotel in central Grozny on July 25, heading for a Chechen press center in the town of Khazaburt in Daghestan.
He has not been seen since.
Mr Shumack had intended to feed his photographs to the Philadelphia Enquirer and the St Petersburg Press.
He has published photos in the "Express-Times," a small-town newspaper in eastern Pennsylvania.
It was the first time he had ventured into a war zone.
Russia's Federal Security Service (FSB) and the American Embassy in Moscow have begun investigations into Mr Shumack's disappearance.
The US Consulate in St Petersburg urges anyone knowing anything about Mr Shumack to contact them at 275-17-01.
Last December another American freelance photographer, Cynthia Elbaum, 28, was killed by a bomb blast in Grozny while taking pictures of air raid damage. (SPP)