Mohammadi, 54, a human rights activist who has spent much of the past two decades behind bars, is believed to have had two heart attacks in recent weeks, according to her supporters.

“We call on the Iranian regime to release her now and give her the care she needs. The world is watching,” Riley Barnes, the US assistant secretary of state in charge of human rights, wrote on social media.

The United States and Israel attacked Iran on February 28. Mohammadi has criticised the war against her country, saying during last year’s Israeli bombing campaign that the violence hurt ordinary people and only entrenched the clerical state.

She was arrested again in December after denouncing the Islamic Republic at a funeral for a lawyer.

The United States under President Donald Trump has not spoken out often on human rights, seeing the issue as a cudgel against adversaries but largely holding off on criticising countries aligned with the Republican administration.