The United States said that a new offer from Iran is "not really responsive" to concerns about its nuclear program, dampening hopes for new talks aimed at breaking a three-year impasse.

It was the most negative reaction so far to the package of proposals submitted by Iran on Wednesday to the United States, China, Russia, Britain, France and Germany, which want Iran to halt its uranium enrichment program.

A US non-profit investigative journalism group, Pro Publica, said it obtained a copy of Iran''s closely-held five-page proposal, in which Tehran said it was prepared to hold "comprehensive, all-encompassing and constructive negotiations."

The talks would address nuclear disarmament as well as a global framework for the use of "clean nuclear energy," according to the document published on Pro Publica''s website, but it did not address Iran''s own nuclear program.

"It is not really responsive to our greatest concern, which is obviously Iran''s nuclear program," Philip Crowley, the assistant secretary of state for public affairs, told reporters.