Iran has threatened to restart deactivated centrifuges and ramp up enrichment of uranium to 20 per cent purity as its next potential big moves away from a 2015 nuclear agreement that Washington abandoned last year.
The threats by Tehran's nuclear agency spokesman would go far beyond the small steps Iran has taken in the past week to nudge stocks of fissile material just beyond limits in the pact.
That could raise serious questions about whether the 2015 agreement, intended to block Iran from making a nuclear weapon, is still viable.
Iran has omitted important details about how far it might go to returning to the status quo before the pact, when Western experts believed it could build a bomb within months.
In a separate stand-off, Iran's foreign minister accused Britain of "piracy" for seizing an Iranian oil tanker last week.
Britain says the ship was bound for Syria in violation of EU sanctions.
Spokesman for Iran's Atomic Energy Organisation Behrouz Kamalvandi confirmed Tehran had enriched uranium beyond the deal's limit of 3.67 per cent purity, passing 4.5 per cent, according to news agency ISNA.
That followed an announcement a week ago that it had amassed a greater quantity of low-enriched uranium than permitted.
UN nuclear watchdog the IAEA said it was still verifying whether Iran had exceeded the 3.67 per cent limit.
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