The future of Iran's Arak heavy water IR-40 reactor is one of the key points in the interim nuclear deal Tehran signed early Sunday in Geneva with world powers.
Washington said Iran agreed to halt progress towards commissioning the reactor that could in theory produce plutonium for use in a nuclear weapon.
The planned start date for the reactor 240 kilometres (150 miles) southwest of Tehran had been unclear.
It would also have to have been operational for at least a year before plutonium could be extracted from spent fuel rods.
Plutonium is an alternative to highly enriched uranium used for an atomic weapon.
If the plant were completed, Iran could extract between five and 10 kilos (10-20 pounds) of weapons-grade plutonium a year from spent rods, enough for one nuclear weapon, experts estimate.
Iran insists it has no nuclear military ambitions, saying Arak will be used to produce medical isotopes and for research.
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"Iran reaffirms that under no circumstances will Iran ever seek or develop any nuclear weapons," the text of the Geneva deal, distributed by the Fars news agency, reads.
The text also said: "Iran announces on concerns related to the construction of the reactor at Arak that for 6 months it will not commission the reactor or transfer fuel or heavy water to the reactor site and will not test additional fuel or produce more fuel for the reactor or install remaining components."
Experts say that Arak has been plagued by construction delays.
UN nuclear watchdog the International Atomic Energy Agency, which monitors Arak, said in August that a planned start-up in the first quarter of 2014 was no longer achievable.
And once on line, the plant would have needed to run for 12-18 months to produce spent fuel that could be used to extract plutonium, said Shannon Kile from the Stockholm International Peace Research Institute (SIPRI).
Iran also does not have a declared reprocessing facility to extract the plutonium, and a secret one would quickly be detected.
"Reprocessing facilities are large and produce radionuclide gaseous products which can be detected by environmental sampling, and that's true whether you have (IAEA) inspectors on the ground or not -- it can be done by airborne means for example," Kile told AFP.
A key point of the Geneva accord is that Iran will "not construct a facility capable of reprocessing. Without reprocessing, Iran cannot separate plutonium from spent fuel," according to the White House.