ISTANBUL— Iran has activated a chain of advanced centrifuges to speed up uranium enrichment in defiance of a 2015 nuclear accord, a senior Iranian official said Saturday, raising the stakes for European powers struggling to prevent the deal's collapse.

Behrouz Kamalvandi, spokesman for Iran's Atomic Energy Organization, announced the new measures at a press conference in Tehran, warning that there was little time left to salvage the deal.

Iranian President Hassan Rouhani said last week that Iran would set aside the accord's restrictions on nuclear-related research and development and would expand its use of advanced centrifuges.

The nuclear agreement was signed by Iran and world powers in 2015 and curbed Iran's nuclear activities in exchange for sanctions relief. President Donald Trump abandoned the accord and reimposed a near-total embargo on Iran's economy in the fall.

France has floated an initiative to grant Iran a $15 billion line of credit to make up for lost oil sales. As part of the agreement, Iran would return to full compliance with the nuclear deal.

But the Trump administration appears unlikely to greenlight the financial package. The U.S. has worked to halt Iran's oil exports, including issuing a warrant for an Iranian supertanker detained by Gibraltar in July.

The administration said the vessel was transporting oil to Syria and was therefore in violation of U.S. sanctions and subject to seizure. Commercial satellite images Friday appeared to show the vessel anchored near the Syrian port city of Tartous.

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