WASHINGTON - The United States called on Tuesday for the release of US citizens held in Iran, but denied a report that Washington had proposed a prisoner exchange for a former US Marine.A lawyer for Amir Hekmati, an Iranian-American former US Marine jailed in Tehran, was quoted in a report earlier on Tuesday on Iran's semi-official Tasnim News Agency as saying that the United States had sought his release through a prisoner swap."Those reports are not accurate," State Department spokesman Jeff Rathke told a regular news briefing in Washington."The US government has not proposed a prisoner exchange for Mr. Hekmati. It's not true. We do however call on the Iranian government to release Amir Hekmati immediately, as well as detained US citizens Saeed Abedini and Jason Rezaian, and to assist us in locating Robert Levinson."Hekmati was arrested in August 2011, his family says, and convicted of spying for the US Central Intelligence Agency, a charge his relatives and the US government deny.Iranian-American Christian pastor Saeed Abedini was sentenced to eight years in prison last year for undermining national security by setting up home-based Christian churches in his native country from 2000 to 2005.Jason Rezaian, an Iranian-American who holds dual citizenship and had been the Washington Post's bureau chief in Tehran, was charged this month after being detained for more than four months.Robert Levinson, a private detective and ex-FBI agent, disappeared during a trip to an Iranian island in 2007. Iranian Foreign Minister Mohammad Javad Zarif said in December last year that there were no traces of Levinson in Iran and that he was not being imprisoned in the country.