HIGHER-GRADE ENRICHMENT

Under the terms of the interim deal, Iran must limit its enrichment of uranium to 20 percent fissile purity for a period of six months as the price for relaxation of some sanctions.

Ali Akbar Salehi, head of Iran's atomic energy organisation, said on Saturday it would have no choice but to step up enrichment to the 60 percent threshold if a bill now moving through parliament is approved - even though it has no current need for such highly enriched uranium.

The bill's supporters say uranium refined to 60 percent concentration would be used to fuel nuclear-powered submarines. That would put Iran on the technical verge of 90 percent fissile purity, which is enough for the core of an atom bomb.

The measure has received expressions of support from at least 218 of parliament's 290 members and, if passed, could threaten progress toward a resolution of the nuclear dispute.

The parliament is much more hawkish than moderate President Hassan Rouhani on the nuclear issue.

But some see the proposal, put forward last month, as a response to a bill introduced in the U.S. Senate that would impose stiffer sanctions on Iran, which Western diplomats fear would shatter the nuclear diplomacy.

Sanctions supporters in the U.S. Congress say the economic penalties have brought Iran to the negotiating table and that keeping up the pressure with more sanctions is the way to ensure that Iran keeps to the November 24 accord and negotiates a comprehensive deal.