U.S. President George Bush says it is unacceptable for Iran to have nuclear weapons. Iran is reportedly doubling its nuclear program, despite previous warnings from Washington as well as the threat of United Nations sanctions.

Iran's semi-official student news agency says nuclear engineers are now operating a second series of uranium centrifuges that have already produced results.

Asked about the reported expansion of Iran's nuclear program, President Bush said the United States must double its efforts to work with the international community to persuade Tehran that there is only further isolation from the world if it continues its nuclear program.

"But whether they've doubled it or not, the idea of Iran having a nuclear weapon is unacceptable," said Mr. Bush. "It is unacceptable to the United States and it is unacceptable to the nations we are working with at the United Nations to send a common message."

Britain, France and Germany have proposed a draft U.N. Security Council resolution that would impose sanctions on Iran for its nuclear program and its refusal to stop enriching uranium.

President Bush says Iran is secretly developing a nuclear weapons program. Iranian President Mahmoud Ahmadinejad says Iran is pursuing a peaceful nuclear program meant to generate electricity.