"Our view is that threats to end cooperation are greatly troubling," deputy State Department spokesman Adam Ereli said.
Iranian leader Ayatollah Ali Khamenei had insisted Iran reserved the right to pull out of the recent deal with the International Atomic Energy Agency.
Ereli also said that Khamenei was reported to have lauded the accord reached in an October 21 meeting with three European foreign ministers, Dominique de Villepin of France, Joschka Fischer of Germany and Jack Straw of Britain.
"We have seen press reports of his remarks praising the agreement made with the European ministers, but at the same time warning that excessive demands from the international community could prompt Iran to end cooperation," Ereli said.
"Threats from Iran to end such cooperation -- rather than give the IAEA full access to and answers about its nuclear activities -- would be gravely troubling and would further deepen the international community's concerns that Iran continues to have something to hide from the IAEA."