LONDON — A British-Iranian woman detained in Tehran since 2016 has been moved to a hospital psychiatric unit, her family said, prompting fears for her well-being.

The moving of the woman, Nazanin Zaghari-Ratcliffe, was initially seen as a positive development, because she had been requesting treatment for months. But her family said on Wednesday that they had not been able to speak with her for days.

They say she is under the watch of the Islamic Revolutionary Guards Corps, a powerful arm of the Iranian military. And they worry that the group’s involvement could mean that she is being pressured by the government.

“Even now it still seems like games of power and control are being played by the Iranian authorities — even at the point of hospitalization,” her husband, Richard Ratcliffe, said in a statement, which said that Ms. Zaghari-Ratcliffe had been moved from prison to the hospital on Monday.