U.S. Defense Secretary Robert Gates says Iran could have the material to develop a nuclear weapon in one to three years.
Gates says the forecast is based on intelligence estimates, and he emphasized it does not include the time it would take Iran to develop an actual weapon and a way to fire it.
He made the comments Friday after a meeting of NATO defense ministers in Brussels.
Gates also said he is disappointed that Turkey voted against new sanctions on Iran at the United Nations Security Council this week. But he said the long alliance between Turkey and the United States remains strong.
The Security Council approved a new round of sanctions against Iran Wednesday for its nuclear program.
Iranian President Mahmoud Ahmadinejad dismissed the U.N. resolution Friday, calling it a "worthless" piece of paper.
Speaking in China, the Iranian leader lashed out at the United States for leading the drive for new sanctions and called the Security Council a U.S. tool.
Mr. Ahmadinejad is in China to visit the Iranian pavilion at the World Expo in Shanghai. He is not expected to meet any senior Chinese officials.
On Wednesday, China joined the U.S., Russia and other permanent members of the U.N. Security Council in backing a fourth round of sanctions against Tehran for engaging in sensitive nuclear activities.
China's Foreign Ministry has said the sanctions resolution was aimed at bringing Iran back to the negotiating table, not blocking further dialogue.
Iran insists its nuclear activities are for peaceful purposes.
Separately, French officials say Russian Prime Minister Vladimir Putin told French President Nicolas Sarkozy that the U.N. sanctions prohibit Russia from selling an air-defense missile system to Iran.
Mr. Putin's remark Friday contradicts Russia's Foreign Ministry, which said earlier that the S-300 surface-to-air missile contract was not covered in the new sanctions.
The U.S. State Department also says the missile system is permissible. But a department spokesman said Thursday the U.S. has appreciated Russia's "restraint" in not delivering the missiles to Iran.
Russia agreed to sell Iran the missiles in 2007 to help Tehran boost its defense capabilities, but Russia has repeatedly postponed the delivery. The delay has angered Iranian officials.
Some information for this report was provided by AP and AFP.