Story highlights Iran may use Americans detained in Iran as leverage in future dealings with the United States

Tehran released five U.S. citizens last month

(CNN) Iran may use detained Americans who were not part of last month's prisoner swap as leverage in future dealings with the United States, America's intelligence chief said Tuesday.

Director of National Intelligence James Clapper's testimony comes after a coalition of Iranian-American groups sent a letter last week to Secretary of State John Kerry, urging the U.S. to do more to secure the release of American businessman Siamak Namazi, who is still being held by Iran.

Tehran released five U.S. citizens last month , including Washington Post reporter Jason Rezaian, Christian pastor Saeed Abedini and Marine veteran Amir Hekmati.

Iran however, "might attempt to use any additional U.S. citizens" held in Iran "as bargaining chips for U.S. concessions," Clapper wrote in a worldwide threat summary submitted to Congress on Tuesday.

Clapper added that Americans detained upon entering Iran, like the 10 U.S. sailors held overnight last month after drifting into Iranian territorial waters, may be used as bargaining pieces as well for Iran "to achieve financial or political concessions in line with their strategic intentions."

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