“That’s her charge,” he added. “She has been lingering in an Australian jail for the past three years.” He said that the United States had requested that the woman be extradited, and that Washington had not responded to the Iranian overtures on a prisoner swap.

Tensions have spiked between Iran and the West after President Trump withdrew the United States from the 2015 nuclear deal and reimposed punishing sanctions last year. In July, Britain and Gibraltar seized an Iranian ship they accused of carrying oil to Syria, and Iran retaliated by detaining a British-flagged tanker in the Strait of Hormuz.

Last month, Australia agreed to join an American-led mission to police the strait against Iranian threats. The fledgling coalition also includes Britain and Bahrain.

The family of Ms. Zaghari-Ratcliffe confirmed that she and the British-Australian blogger were being held in the same ward at Evin Prison. The blogger came out of solitary confinement a few weeks ago, the family said.

Ms. Zaghari-Ratcliffe, 41, had lived in London for more than a decade before she was detained in Tehran while trying to return to Britain; she had been in Iran visiting family with her young daughter.

At the time, she was a program director at the Thomson Reuters Foundation — a charity independent of the media conglomerate Thomson Reuters. She was accused of plotting to overthrow Iran’s government. Her family and the foundation have vigorously denied the charge.

Ms. Zaghari-Ratcliffe’s husband, Richard Ratcliffe, said that the British Foreign Office had not given the family information about the other British nationals being held, but that the families and their representatives in Parliament were pressing for a joint meeting with Dominic Raab, Britain’s foreign secretary.