This would be Iran's second breach of the deal in as many weeks, diplomats familiar with the figures said.
The tanker's detention comes at a particularly sensitive time as tensions between the United States and Iran grow over the unraveling of the 2015 nuclear deal, from which President Donald Trump withdrew a year ago.
Iran has said it will boost its uranium enrichment in a few hours above a cap set by the nuclear deal, a move that could mean the return of all economic sanctions on Tehran.
At Wednesday's meeting, IAEA officials said Iran's stock of enriched uranium was now 213.5 kg.
"Iran has always been secretly 'enriching, ' in total violation of the awful 150 Billion Dollar deal made by John Kerry and the Obama Administration".
"There is no credible reason for Iran to expand its nuclear program, and there is no way to read this as anything other than a crude and transparent attempt to extort payments from the global community", said a Trump administration statement issued at the closed-door session of the IAEA board in Vienna.
President Trump later alleged in a tweet that Iran had been "secretly "enriching" uranium in total violation" of the deal made by his predecessor, Barack Obama, and promised that sanctions would "soon be increased, substantially".
The president also called out what he alleged was United States hypocrisy over uranium enrichment when the USA is the only country in history to have ever deployed nuclear weapons in an offensive capacity.
After the US' withdrawal, the European signatories of the deal said they were still trying to keep the accord alive by launching the long-awaited INSTEX to help Iran reap some benefits from the deal. Iran has conditioned any talks on first being able to export as much oil as it did before the USA withdrawal.
US President Donald Trump warned Wednesday that sanctions against Iran would soon be "increased substantially", after Tehran said it had exceeded a limit on enriched uranium reserves under a 2015 nuclear deal abandoned by Washington. He says the Trump administration hasn't learned.
The issue had also been raised in Iranian President Hassan Rouhani's letter to P4+1 and in officials' remarks and statements, he said. That has been a political nonstarter in Washington, meaning that the prospect of negotiations between Washington and Tehran remains bleak. The next day, its atomic energy agency announced that it was enriching uranium to 4.5 percent.
France and other countries have called on Iran to go back to complying with the nuclear deal.
The US and the European countries are deceiving us.
Iranian President Hassan Rouhani, a key ally of Assad, said that the United Kingdom was "an initiator of insecurity and you will understand its repercussions".
Last week, authorities in Gibraltar intercepted an Iranian supertanker that was believed to be breaching European Union sanctions by carrying a shipment of Tehran's crude oil to war-ravaged Syria.
Rouhani said that Iran's moves were within the framework of the deal and he rejected a warning from European parties to the pact to continue its full compliance.