France urged Iran on Wednesday not to scale back further on its commitments to a 2015 nuclear deal, saying Tehran's new threat to speed up uranium enrichment next month was "especially worrying".
Iran is breaching restrictions of the pact with major powers step-by-step in response to tough sanctions imposed by the United States, which pulled out of the deal last year.
Tehran has said its next move would be taken on November 6 and diplomats fear this could force a response from European powers, who have been trying to salvage the accord. Britain, France and Germany, all signatories, have refrained from acting so far.
"Iran must abstain from crossing an especially worrying new phase of new measures that could contribute to an escalation in tensions," French Foreign Ministry spokeswoman Agnès von der Muhll told reporters in a daily briefing, according to Reuters.
Iranian President Hassan Rouhani said on Monday that Tehran was working on advanced IR-9 centrifuges for uranium enrichment.
The nuclear deal only lets Iran accumulate enriched uranium with just over 5,000 of its first-generation IR-1 centrifuges at Natanz. It lets Iran use small numbers of more advanced models for research, without producing enriched uranium.
Iran is now enriching uranium with advanced centrifuges and installing more that should come online in coming weeks.
Rouhani's remarks suggested Iran was developing a new centrifuge, the IR-9, violating the deal which specifies the centrifuges Iran can use and making no mention of an IR-9.
French President Emmanuel Macron attempted and failed last month to broker talks between US President Donald Trump and Rouhani in New York. Prospects of any talks in coming weeks seem slim with Tehran demanding US sanctions are lifted first.
The nuclear deal aimed to extend the so-called "breakout time" Iran would need to obtain enough fissile material for a bomb, if it sought one, to a year instead of two or three months.
"November 6 will be Iran's fourth violation. Until now they have been political and symbolic with a limited impact on the breakout time, but the more they violate, the less choice and latitude they have that doesn't impact the breakout time," said a French diplomatic source.
"After November, the world doesn't end, but it becomes much harder to save the deal," the source added.
Iran says it has enriched uranium for civilian purposes and has never sought nuclear weapons, but the United States and IAEA believe it once had a nuclear weapons program that it ended.
"Iran is underscoring that it will no longer be hemmed in by the nuclear agreement, nor is it particularly alarmed by the increasingly concerned statements coming out of Europe," Eurasia's Iran analyst Henry Rome said.
"This is a recipe for a significant nuclear escalation in early November, not just another incremental step."