The step comes as the US builds up its military near Iran and threatens to strike over concerns led by its disputed nuclear programme.

“The Iranian regime must stop taking hostages and release all Americans unjustly detained in Iran, steps that could end this designation and associated actions,” secretary of state Marco Rubio said in a statement.

If Iran makes no progress, Rubio said that the US could eventually decide that US passports are invalid for travel to Iran.

The United States only forbids its nationals from traveling to one country — North Korea — and many Iranian-Americans routinely travel to Iran, ruled since 1979 by clerics after the overthrow of the pro-US shah.

President Donald Trump in September signed an executive order that created the blacklist on wrongful detention, similar to designations by the US on terrorism.

Iran becomes the first country to be placed on the blacklist. The Islamic republican has detained a number of Americans in recent years, usually dual nationals whom the government considers Iranian citizens.

“No American should travel to Iran for any reason. We reiterate our call for Americans who are currently in Iran to leave immediately,” Rubio said.