Iran's offer offer to halt production of low enriched uranium is not credible because the Islamic republic has a record of making "empty promises," the United States said Friday.

Iranian President Mahmoud Ahmadinejad told The New York Times last week that his country had offered to stop its production of low enriched uranium, which can be a stepping stone to produce atomic weapons, provided the West gives it the nuclear material.

"Ahmadinejad makes a lot of empty promises," State Department spokeswoman Victoria Nuland told reporters.

"He knows exactly what has to happen. If Iran has a serious proposal to put forward, it has to put it forward to the IAEA."

The UN nuclear watchdog, or International Atomic Energy Agency, and the United States "can study it and then we can respond, but from our perspective at the moment, this looks like a diversion from the real issue," Nuland said.

Ahmadinejad said Iran wanted 20 percent enriched uranium for a medical reactor that makes isotopes for cancer treatment.

Western countries say Iran already has enough uranium for the reactor and that any additional uranium would be used to make weapons.

Accused by Western nations of seeking to develop a nuclear weapon, Tehran is under four sets of UN sanctions for refusing for years to bow to international demands to rein in uranium enrichment.

Iran denies the charges, saying its nuclear program is purely for peaceful purposes.