TEHRAN, Iran — Iran was poised Thursday to begin work on advanced centrifuges that will enrich uranium faster as the 2015 nuclear deal unraveled further, and a last-minute French proposal offering a $15 billion line of credit to compensate Iran for not being able to sell its crude oil abroad because of US sanctions looked increasingly unlikely.
Meanwhile, Iran released seven crew members from a detained British-flagged oil tanker Stena Impero in a goodwill gesture and the mariners flew out of Iran, the ship’s owner said.
Iran has yet to say officially what exact steps it will take if Europeans fail to salvage the deal, which is set to expire Friday. Centrifuges that speed enrichment further shorten the time Tehran would need to have enough material available to build a nuclear weapon — if it chose to do so.
Under the deal, which has steadily unraveled after President Trump’s unilateral withdrawal of the United States from the accord last year, experts thought Iran would need about a year to reach that point.
Iran’s atomic energy agency was to make an announcement on Saturday detailing its next step, which President Hassan Rouhani described as highly significant, according to the semi-official Tasnim news agency and other Iranian media. The details would be unveiled at a press conference in Tehran, the reports said.
The United States has continued its effort to choke off Iran’s crude oil sales abroad, a crucial source of government revenue. Iranian Foreign Minister Mohammad Javad Zarif insists his country will do everything it can to keep those sales going, though he described US sanctions in an angry tweet Thursday as the equivalent of a ‘‘jail warden.’’
‘‘We will sell our oil, one way or the other,’’ Zarif told Russian broadcaster RT in a recently aired interview. ‘‘The United States will not be able to prevent that.’’
Tensions between Iran and the US have been growing since Trump’s pullout from the nuclear deal, which saw Tehran agree to limit its uranium enrichment in exchange for the lifting of economic sanctions. Trump subsequently re-imposed old sanctions on Iran and created new ones, going as far as targeting Iranian officials like Zarif and Iran’s paramilitary Revolutionary Guard.
Meanwhile, mysterious oil tanker attacks struck near the Strait of Hormuz in recent weeks, attacks that the US blames on Iran. Tehran denies it was involved.
Iran also shot down a US military surveillance drone and seized oil tankers as America deployed nuclear-capable B-52 bombers, advanced fighter jets, and more troops to the region.
The United States has sought to seize an Iranian oil tanker, the Adrian Darya-1, now thought by analysts to be off the Syrian coast despite a pledge by Tehran that its cargo wasn’t bound there.
In his speech late Wednesday, Rouhani said Tehran would soon begin work on research and development of ‘‘all kinds’’ of centrifuges that enrich uranium by rapidly spinning uranium hexafluoride gas.
Iran has begun to break limits of the deal, such as creeping beyond its 3.67 percent enrichment limit and its stockpile rules. Using advanced centrifuges speeds up enrichment and Iranian officials already have raised the idea of enriching to 20 percent — a small technical step from weapons-grade levels of 90 percent.
Iran long has maintained its nuclear program is for peaceful purposes and denies it seeks an atomic bomb. However, Western nations have pointed to previous Iranian research into a weapons program that UN experts say largely ended in 2003.
France in recent days had pushed the idea of offering Iran a $15 billion credit to sell its oil, though details remain unclear and it appeared the deal wouldn’t come through before Iran’s deadline Friday.
That appears to show Iran trying to resort to its own maximum pressure campaign through the nuclear program to get what it wants, said Henry Rome, an analyst for the Eurasia Group.
‘‘Iran’s plan appears to be provocative but reversible,’’ Rome said. ‘‘Tehran is building leverage, not a bomb.’’
Israeli Prime Minister Benjamin Netanyahu, a longtime hawk on Iran, called on the world to increase its pressure on Tehran before flying to London for meetings with British officials on Thursday.
While Trump maintains he’s open for North Korea-style talks with Iran, his administration has continually upped its pressure on the Islamic Republic. On Wednesday, the US imposed new sanctions on an oil shipping network it alleged had ties to the Guard and offered up to $15 million for anyone with information that disrupts the Guard’s operations.
‘‘There will be more sanctions coming,’’ Brian Hook, the US special envoy for Iran, told reporters at the State Department. ‘‘We can’t make it any more clear that we are committed to this campaign of maximum pressure.’’
Hook also directly emailed or texted captains of Iranian oil tankers, trying to scare them into not delivering their cargo, according to the Financial Times.
Zarif reacted angrily to the report.
‘‘Having failed at piracy, the US resorts to outright blackmail — deliver us Iran’s oil and receive several million dollars or be sanctioned yourself,’’ the diplomat wrote on Twitter.