The Russian Imperial Movement is not considered to be sponsored by the Russian government, officials said, although President Vladimir V. Putin has tolerated its activities and it has helped advance the Russian government’s external goals by recruiting Russian fighters to aid pro-Russia separatists in eastern Ukraine.

The group has also helped support neo-Nazi organizations in Scandinavia, which dovetails with the Russian government’s broader pattern of trying to stoke internal divisions, including along racial lines, and sow chaos in Western democracies.

In 2017, the Russian Imperial Movement came up at a trial in Sweden of three men who were accused of plotting bomb attacks targeting asylum seekers. Prosecutors said two of the defendants had traveled to St. Petersburg, Russia, to attend 11 days of paramilitary training at a camp operated by the group, fueling their radicalization.

Mr. Sales said the group operated two facilities in St. Petersburg that offered paramilitary training to neo-Nazis and white supremacists. He said the United States had assessed that the camps were “likely being used for woodland and urban assault, tactical weapons and hand-to-hand combat training.”

Mary McCord, a former head of the Justice Department’s National Security Division, said that until now, the counterterrorism sanctions system had overwhelmingly been used against Islamist extremist groups. She called its expansion to a white supremacist group significant.

“It is important,” she said. “Far-right extremist causes, in particular white supremacy and white nationalism, have become more international. It is appropriate for the State Department to have been scrutinizing whether there are organizations that meet the criteria for that designation because with it, the organization becomes poison in terms of doing business with it or providing funds, goods or services to it.”

The move is also another example of the Trump administration expanding its use of the power Congress granted to the executive branch to impose sanctions on groups by designating them foreign terrorist organizations. Last year, under a separate authority, the United States designated the Islamic Revolutionary Guards Corps, an arm of the Iranian military, as a foreign terrorist group — the first time it gave that label to a nation-state entity.