Reporters Without Borders has expressed deep concern about Iranian opposition leader MirHosein Mousavi, who was hospitalized on Thursday to undergo heart surgery.

The press rights group issued a statement on Friday, writing: “The conditions of Mousavi’s house arrest have damaged his health and he must be released at once. We urge the Supreme Leader, Ayatollah Khamenei, to act quickly to avoid exposing him to further danger. We will hold Khamenei responsible for anything that happens to Mousavi or to the many other journalists who are being kept in detention despite their poor health.”

Mousavi has been under house arrest together with his wife, Zahra Rahnavard, since February of 2011. Reports from Iran indicate that Mousavi, who was taken to hospital with heart complications, underwent angiography and treatment for a blocked artery. He has now reportedly been released from the hospital and returned to house arrest.

Reporters Without Borders said: “Holding political prisoners incommunicado is tantamount to enforced disappearance and violates international law. The Iranian authorities nonetheless resort to such methods systematically with journalists and netizens.”

A number of Iranian activists have called on leaders attending the NAM Summit in Tehran to demand a visit with the two opposition leaders, MirHosein Mousavi and Mehdi Karroubi, who have been under house arrest for more than 18 months.

A letter written by Mousavi supporters addressed to Egyptian President Mohammad Mursi urges him to inquire about the health of MirHosein Mousavi.