The United States, which is seeking international support to impose new sanctions against Iran, dismissed on Wednesday Iranian President Mahmoud Ahmadinejad's saying that the nuclear issue of the Islamic republic was closed.

"I am sorry to tell President Ahmadinejad that the case is not closed," U.S. Under Secretary of State for political affairs Nicholas Burns told reporters a day after Ahmadinejad demonstrated Tehran's determination to keep its controversial nuclear program going.

"In our opinion, the nuclear issue of Iran is now closed and has turned into an ordinary (International Atomic Energy) Agency matter," the Iranian leader told the United Nations General Assembly.

Contrary to Ahmadinejad's remarks about nuclear disputes, Burns said, "We're going to keep going," he said. "If Mr. Ahmadinejad thinks somehow that he has been given a pass, he is mistaken about that."

The United States has long accused Iran of trying to develop nuclear weapons under the cover of a civilian nuclear program. Iran has repeatedly denied the U.S. charges and insists that its nuclear program is for peaceful purposes only.

Since December 2006, Iran has been subject to two United Nations sanctions resolutions as well as a growing number of financial penalties from individual nations.