Iranian lawmakers presented a bill to the parliament's presiding board which could oblige the government to enrich uranium to 60 percent if new sanctions are imposed, media reported Wednesday.
"If the other negotiating parties ratchet up the sanctions, impose new sanctions or violate our country's nuclear right, this bill will immediately oblige the government to... launch the Arak heavy water reactor and produce 60 percent enriched uranium," ISNA news agency quoted lawmaker Mehdi Mousavi-nejad as saying.
Under the landmark deal struck on November 24, Iran agreed to roll back or freeze parts of its nuclear drive for six months in exchange for modest sanctions relief and a promise by Western powers not to impose new sanctions.
Under the accord, Iran pledged to limit uranium enrichment to 5 percent. It will also neutralise its stockpile of medium-enriched material, which is relatively easy to convert to the weapons-grade level of 90 percent or above.
Iran is also committed not to make further advances at its Fordo, Natanz and Arak facilities.
The US administration hopes the interim agreement can buy time for the negotiation of a comprehensive accord and build trust on both sides, but several US lawmakers have called for tougher sanctions, arguing that they can wring more concessions from Tehran.
The Iranian bill, signed by 100 parliamentarians, was presented to the presiding boards but has yet to be approved by the 290-member parliament.
A small heavy water research reactor at the Arak site is of concern because Tehran could theoretically extract weapons-grade plutonium from its spent fuel if it also builds a reprocessing facility. Iran agreed not to build such a facility.
Western powers suspect Iran's nuclear activities mask military objectives, despite repeated denials from Tehran, which insists its programme is entirely peaceful.
Experts from Iran and the P5+1 -- Britain, China, France, Russia and the United States plus Germany -- have been holding difficult negotiations on the implementation of the nuclear accord.
The negotiations have been temporarily suspended because of Christmas holidays.