Iranian authorities have arrested the elderly father of an American who has been in jail in Iran for the past four months, the man's family said.

Baquer Namazi, 80, was arrested on Monday in Tehran, his wife Effie Namazi said on Facebook.

Mr Namazi's son, Siamak Namazi, a dual US-Iranian citizen, was detained by Iran's Islamic Revolutionary Guard in October while visiting family.

Officials have yet to announce charges against Siamak.

Baquer Namazi, who is also a dual US-Iranian citizen, was taken to Evin Prison, where his son is also being held, Mrs Namazi said in her post.

He has a serious heart and other conditions which requires special medication, she said.

"Now both my innocent son Siamak and my Baquer are in prison for no reason. This is a nightmare I can't describe," she wrote.

"I have been trying to find out more information but have been unable to do so. The lawyer also couldn't get any information or get to see him."

Asked at a Senate hearing about the elder Mr Namazi's arrest, US Secretary of State John Kerry said: "I am very familiar with this and I am engaged on it specifically, but I am not permitted due to privacy reasons to go into details here."

US State Department Spokesman Mark Toner said the agency was aware of reports that a US citizen had been detained in Iran.

A former Iranian provincial governor and Unicef official, Baquer Namazi was more recently running Hamyaran, an umbrella agency for Iranian non-governmental organisations.

His son Siamak was most recently working for Crescent Petroleum in the United Arab Emirates, and previously headed a consulting business in Iran.

Baquer Namazi's arrest comes more than a month after a high-profile prisoner swap between Iran and the United States, which saw five Americans released from Iranian prison in return for clemency for seven Iranians.

Three of the four freed US-Iranian prisoners, including Washington Post reporter Jason Rezaian, left Iran late last month. A fourth did not fly with them, US officials said.