Amnesty International said Wednesday that Russian authorities have closed off its Moscow office.
The human rights group's Europe and Central Asia director, John Dalhuisen, called it "an unwelcome surprise for which we received no prior warning."
When staffers arrived for work Wednesday morning, they found a seal over the door declaring that they were not allowed to enter without a city official. Amnesty said it rented the property "directly from the city's municipal authorities" and that "the locks and alarm system had been removed and the electricity supply appeared to have been cut off."
The human rights group's Moscow director, Sergei Nikitin, posted photos to Facebook showing the seal, and broken locks thrown on the ground.
Amnesty said it has not been able to reach municipal authorities for further information. According to the BBC, a spokesman for President Vladimir Putin told reporters that "he had no information about what had happened."
"Given the current climate for civil society work in Russia, there are clearly any number of plausible explanations, but it's too early to draw any conclusions," Dalhuisen said in the statement. "We are working to resolve the situation as swiftly as possible and very much hope there is a simple administrative explanation for this setback to our work."
He added that Amnesty is "100% confident that we fulfilled all our obligations as tenants."
NPR's Lucian Kim reported that "organizations that criticize the Russian government are under increasing pressure." He added that "those with funding from abroad have been branded 'foreign agents.' "
Amnesty routinely criticizes Russia over human rights abuses, Lucian noted, including a recent report in which it said Russia might have committed war crimes in Syria.
The group's December 2015 report said that "the Russian armed forces appear to have directly attacked civilians or civilian objects by striking residential areas with no evident military objective and even medical facilities, resulting in deaths and injuries to civilians."
Amnesty staffer Ivan Kondratenko said that "nothing of this sort has ever happened in the 20 years that Amnesty International has been renting its office space in Moscow," according to the independent news site Meduza.
State Department spokesman John Kirby told reporters Wednesday that the U.S. is concerned and is monitoring the situation.