Alcee Hastings, who chairs the U.S. Helsinki Commission tasked with monitoring human rights, said in a statement that the OSCE's Spanish chairmanship "sent the wrong message" by prohibiting the Russian-Chechen Friendship Society from attending the September 13-14 conference in Vienna.
The decision prompted a walkout by the U.S. delegation attending the conference, with members of the team accusing the OSCE of bowing to pressure from Russia.
"Let the facts speak for themselves," Hastings' statement read. "Spain gave in to Russian strong-arm tactics barring a nonviolent NGO from attending this meeting."
The U.S. mission in Vienna says it will lodge a formal protest with the OSCE.
Oksana Chelysheva, one of the heads of the Russian-Chechen Friendship Society, told RFE/RL's North Caucasus Service that her organization had intended to participate in the conference, and received no formal notification from the OSCE that it had not been registered.
Chelysheva claims the Spanish chairmanship made the decision at the recommendation of the Russian delegation, and did not consult with the delegations from other countries. "This has been proven already," she said. "The first day of the conference opened precisely with a direct question, posed by several national delegations, to the chairman's delegation from Spain, demanding an explanation of the real reasons why our organization was refused registration."
A spokeswoman for the OSCE's Office for Democratic Institutions and Human Rights, Urdur Gunnarsdottir, said the OSCE's Spanish leadership had put the registration of the Russian-Chechen Friendship Society on hold because of "serious objections by Russia."
According to Gunnarsdottir, the Russian side claimed the member of the group who was due to attend the conference was a supporter of Chechen separatists.
The Russian-Chechen Friendship Society has published documents alleging that Russian forces and their pro-Moscow Chechen allies are guilty of torture, abductions, and the murders of Chechen civilians.
In October, the highest court in the Nizhny Novgorod region in central Russia found the group guilty of extremism and ordered its activities be ceased. The ruling was later upheld by Russia's Supreme Court.
Rights campaigners have protested the group's closure, calling it an attempt by the Kremlin to silence critical voices.