GENEVA — Russia has committed “multiple and grave” human rights abuses in Crimea since reclaiming the territory three years ago, the United Nations said on Monday, including arbitrary arrests, torture and the imposition of Russian citizenship on residents of the former Ukrainian region.

United Nations investigators have documented evidence of arbitrary detentions, torture and abductions since Russian troops annexed the Crimean Peninsula, according to a report presented in Geneva on Monday and linking abuses to the Federal Security Service, Russian police officers and a paramilitary group.

The abuses included the extrajudicial killing of at least one pro-Ukrainian activist, the panel found, and while dozens of people abducted from 2014 to 2016 have been released, at least 10 are still missing. Many of the rights violations took place in 2014, shortly after Crimea’s annexation in March that year, the report said, but arbitrary arrests are still reported regularly and appear to seek to instill fear and stifle opposition.

“The frequency and severity of these human rights violations, together with the lack of accountability, has created an atmosphere of impunity which encourages the further perpetuation of such violations,” said Fiona Frazer, who led the investigating mission, which the United Nations General Assembly ordered in December.