VIENNA • Iran is set to breach a cap on its enriched-uranium stockpile within days, potentially pushing its conflict with the United States into a dangerous new phase.
Limiting the volume and purity of its accumulated uranium was a central part of Iran's nuclear deal with world powers in 2015.
The deal was designed to prevent Iran from breaking out and constructing a weapon within a year. The US abandoned the deal in May last year and reimposed sanctions.
President Donald Trump said on Saturday - days after Iran shot down a US Navy drone - that he will impose major additional US penalties today, without providing details.
In his Twitter post, Mr Trump specified the need to prevent Iran from acquiring nuclear weapons, echoing comments made to reporters at the White House on Saturday.
"All I want is no nuclear weapons," Mr Trump said.
"Let me just tell you, the are not going to have a nuclear weapon."
Iran eliminated some 97 per cent of its enriched uranium to comply with the nuclear agreement with China, France, Germany, Russia, Britain and the US. The country previously had enough material to build more than a dozen bombs.
In a move foreshadowed by Iranian leaders for weeks, the cap set on the country's stockpile of enriched uranium could be broken by Thursday, a day before negotiators from the countries still committed to the accord - mostly European - meet in Vienna.
"If Iran's leadership comes to the conclusion that it has no choice other than talking to Washington, it will do so only after it has resuscitated its leverage," said Mr Ali Vaez, a director at the International Crisis Group.
"This means that the path to new negotiations passes through another perilous nuclear stand-off."
Iran eliminated some 97 per cent of its enriched uranium to comply with the nuclear agreement with China, France, Germany, Russia, Britain and the US.
The country previously had enough material to build more than a dozen bombs.
While Iran has always said its programme is civilian, world powers pursued the deal because they doubted that claim.
Iranian President Hassan Rouhani signalled on May 8 that the country would soon violate terms of the agreement unless European governments, which have not pulled out of the deal, guarantee the trade it envisages.
Five weeks later, Iran said that it would increase the rate of enrichment.
Barring policy change or mechanical breakdown, Iran could accumulate the volume of material needed to build a weapon by the end of the year.
Mr Daryl Kimball, executive director of the Arms Control Association, a Washington non-profit organisation, said: "While Iran's frustration with Trump's reckless pressure campaign is understandable, we strongly urge Iran to remain in compliance with the nuclear deal."
Meanwhile, the Trump administration's policy towards Iran has "increased the risk of a new nuclear crisis", Mr Kimball said.
The association estimates that Iran would need about 1,050kg of uranium enriched to 3.67 per cent to build one bomb.
The material would then need to undergo further enrichment.
International Atomic Energy Agency monitors said last month that Iran has met its obligations.
Diplomats from the countries remaining in the accord will meet on Friday to discuss "Iran's announcement regarding the implementation of its nuclear commitments".