Nuclear talks are underway between Iran and delegates from the United States and five other world powers, including the European Union, Russia, Britain, France, and Germany.
The talks are the first with Iran in more than a year and are meant to urge Tehran to stop enriching uranium.
Iran claims the enrichment is for fuel purposes, but it's widely believed they are building nuclear weapons.
"Given that Iran's own supply of uranium is not enough for a peaceful nuclear energy program, this calls into further question Iran's intentions and raises additional concerns at a time when Iran needs to address the concerns of the international community," Mike Hammer, spokesman for the U.S. National Security Council, said.
The United Nations is demanding Iran stop parts of its nuclear program. Neither the U.S. nor Israel have ruled out military action if Tehran refuses to halt its program.
Before the talks began, Iranian President Mahmoud Ahmadinejad said, "Iran has not and will not allow anybody in the talks to withdraw one iota of the rights of the Iranian nation."