Nuclear experts from Iran and six world powers will resume talks on how to roll out last month's landmark deal on Monday in Geneva, hoping to resolve numerous technical issues before the accord can be put into place.
A spokeswoman for EU foreign policy chief Catherine Ashton, who oversees diplomacy with Iran on behalf of the United States, Russia, China, France, Britain and Germany, said talks were scheduled to last one day for now.
Two rounds of negotiations have been held so far since Iran agreed on November 24 to curb its most sensitive nuclear work in return for relief from some economic sanctions that are damaging its oil-dependent economy.
The experts have to work out when the deal will be implemented, triggering the loosening of economic restrictions by the European Union and United States.
A key sticking point appears to be what information Western governments will receive in advance to verify that Iran is meeting its end of the deal before they lift some sanctions.
Other outstanding issues address how exactly sanctions will be eased and practical details of Iranian concessions. Some diplomats from the six nations have said they hoped the deal could be put fully in place by the second half of January. The talks resume at a sensitive time.
A group of 100 hardline Iranian lawmakers are seeking to oblige moderate President Hassan Rouhani's government to increase uranium enrichment to 60%, a level that can produce bomb-grade material if enriched further, if new sanctions are imposed on the Islamic Republic.
It is not clear if the bill would be debated in Iran's 290-seat parliament, as the country's most powerful authority Supreme Leader Ayatollah Ali Khamenei has repeatedly backed the Geneva talks. If approved, the bill has to be ratified by a constitutional watchdog body to become law.
Iranian lawmakers said the measure was a response to "America's hostile measures", referring to a legislation introduced by 26 US senators last week to impose new sanctions on Iran if the country breaks the Geneva interim deal.
An Iranian official warned that the process could be derailed if US lawmakers imposed the tougher curbs despite the Obama administration's opposition.
"Any wrong move by the US Congress in the form of approving a sanctions bill targeting officials of the Islamic Republic of Iran will be given a similar response by the parliament," deputy head of the Iranian parliament's National Security and Foreign Policy Committee Mansour Haqhiqhatpour said, Press TV reported.
"Ratification of such a bill could put an end to nuclear negotiations, and Tehran may opt not to continue negotiations in the wake of such sanctions." The proposed US legislation would require reductions in Iran's petroleum production and apply new penalties to Iran's engineering, mining and construction industries.
Iran rejects Western fears that its nuclear work has any military intentions and says it needs nuclear power for electricity generation and medical research.
The November 24 agreement is meant to give the six powers time to negotiate a final settlement with Iran that will put an end to the decade-old standoff and ease worries over a new war in the Middle East.
(Reporting by Justyna Pawlak; Additional reporting by Parisa Hafezi in Ankara; Editing by Toby Chopra)