Iranian officials ratcheted up pressure before a weekend deadline for European nations to come up with a solution for Iran to sell its oil abroad after the United States withdrew from a 2015 nuclear deal and reimposed devastating sanctions.
President Hassan Rouhani threatened on Wednesday to take additional steps and accelerate its nuclear activities if Europe fails to provide a solution by Friday.
"Iran's third step is of an extraordinarily significant nature," Rouhani said, without detailing what that would entail, adding a "decree will be announced today or tomorrow".
"I see that it's unlikely a conclusion will be reached with Europe today or tomorrow," he said, suggesting European powers have another two months to resume talks to save the landmark nuclear accord.
"They know what we want, and we know what they want."
A short time after Rouhani's statement, US officials announced new sanctions on Iran, this time targeting a shipping network it said was run by Iran's Revolutionary Guard to allegedly smuggle oil.
In two separate moves already, Iran reduced its commitments outlined in the deal and vowed further measures unless the European signatories of the pact did more to protect Tehran from the effects of the US "maximum pressure" sanctions campaign.
The nuclear deal - agreed on by Iran, Britain, China, France, Germany, Russia and the US - offered Tehran relief from many international sanctions in exchange for accepting curbs on its nuclear programme.
Since Washington pulled out of the deal last year and reimposed sanctions, Iran has insisted it wants to save the pact but has demanded the remaining signatories - especially the Europeans - provide additional economic support.
The statements came as Iranian officials appeared to give mixed signals in response to a French proposal to save the agreement by offering Iran about $15bn in credit lines until the end of the year if Tehran comes fully back into compliance.
Iranian officials initially said they were considering the French plan when news of it emerged on Tuesday. Deputy Foreign Minister Abbas Araqchi appeared to back its main terms on Wednesday.
"Our return to the full implementation of the nuclear accord is subject to the receipt of $15bn over a four-month period, otherwise the process of reducing Iran's commitments will continue," the semi-official news agency Fars quoted Araqchi as saying.
"Either Europe has to buy oil from Iran or provide Iran with the equivalent of selling oil as a credit line guaranteed by Iran's oil revenues, which in some sense means a pre-sale of oil," he added.
Soon after, Iran's English-language Press TV issued a short report stating: "Iran has rejected a $15bn loan offered by EU," without giving further details. Western and Iranian sources had described the French plan as the offer of a credit line, not a loan, although the precise details have not been made public.
Brian Hook, the US State Department coordinator on Iran, ruled out the French-proposed credit line.
"We did sanctions today. There will be more sanctions coming. We can't make it any more clear that we are committed to this campaign of maximum pressure and we are not looking to grant any exceptions or waivers," said Hook.