WASHINGTON — The Obama administration has gone to court to protect the files of an influential anti-Iran advocacy group, saying they likely contain information the government does not want disclosed.

The highly unusual move by the Justice Department raises questions about the connections between the American government and the group, United Against Nuclear Iran, a hard-line voice seeking to prevent Iran from obtaining nuclear weapons. The group has a roster of prominent former government officials and a reputation for uncovering information about companies that sometimes do business with Iran, in violation of international sanctions.

The Justice Department has temporarily blocked the group from having to reveal its donor list and other internal documents in a defamation lawsuit filed by a Greek shipping magnate the group accused of doing business with Iran. Government lawyers said they had a “good faith basis to believe that certain information” would jeopardize law enforcement investigations, reveal investigative techniques or identify confidential sources if released.

Judge Edgardo Ramos of the United States District Court for the Southern District of New York called the government’s involvement “very curious.” But since March, he has agreed not to force the group to reveal the information. The government has until Thursday to say whether it will formally claim law enforcement privilege and try to keep the information secret permanently.