The United States rejected on Wednesday Iran's claim that its nuclear program is a "closed" matter and said it would keep pushing for fresh U.N. Security Council sanctions that are resisted by China and Russia.

The West accuses Iran of seeking to develop atomic weapons under the cover of a civil nuclear program. Iran denies this, saying its program is to produce power so that it can export more of its valuable gas and oil.

Iranian President Mahmoud Ahmadinejad said on Tuesday the issue of his country's nuclear ambitions was "closed" and a matter to be handled by the U.N. watchdog, the International Atomic Energy Agency.

"The case is not closed," U.S. Under Secretary of State Nicholas Burns told reporters. "He is completely mistaken and the international community is not going to allow him to forget about the fact that his country is operating against the wishes of the Security Council."

The U.N. Security Council has demanded that Iran suspend its uranium enrichment, a process that can produce material for nuclear weapons, and has imposed two sanctions resolutions against Tehran for its failure to do so.

If Iran were to suspend uranium enrichment, the United States has offered join talks with Tehran led by the so-called EU3 -- Britain, France and Germany.

"With the failure of the Iranian government ... to accept that offer, we have no other alternative but to continue the sanctions and I have not see a lessening of commitment on the part of the majority of our partners," Burns said.

Burns did not address directly the misgivings that China and Russia about passing a third sanctions resolution now but conceded that it would be "some time" before agreement on a fresh resolution might be reached.

Burns said the political directors of the five permanent U.N. Security Council members -- Britain, China, France, Russia and the United States -- would meet to discuss a third resolution over dinner on Wednesday.

The group, which met in Washington last Friday, is expected to hold talks again on Thursday, he said, in preparation for a meeting by their ministers on Friday.