GENEVA — A Russian diplomat at the United Nations Human Rights Council in Geneva has been accused of sending a threatening message on Twitter to a member of a human rights organization that supports activists in Russia, according to rights activists.

The intimidating message was sent in March to Florian Irminger, the head of advocacy for the Human Rights House Network. It showed an image of a flayed and bloody body from the popular television show “Game of Thrones” and said, “Our blades are sharp.”

The Council president, Choi Kyong-lim of South Korea, denounced the message as “completely unacceptable,” and told the Council during its opening session on Monday that he had taken up the issue with the head of an unnamed mission.

An investigation traced the message to a diplomat seated at Russia’s place in the Council during a discussion of a resolution promoting the protection of human rights defenders, Mr. Irminger said.