Vienna — The UN's nuclear watchdog confirmed Monday that Iran has exceeded a limit on its enriched uranium reserves set by a 2015 nuclear deal.

The International Atomic Energy Agency "verified on July 1 that Iran's total enriched uranium stockpile exceeded 300 kilograms", a spokesperson said, shortly after Tehran announced it had crossed the limit in retaliation to new US sanctions.

British foreign minister Jeremy Hunt said he was "deeply worried" after Iran said it had amassed more low-enriched uranium than permitted under its 2015 agreement with major powers, but said that Britain still backed the deal.

"Deeply worried by Iran's announcement that it has broken existing nuclear deal obligations. UK remains committed to making deal work and using all diplomatic tools to de-escalate regional tensions," Hunt said in a tweet after Iran's announcement.

"I urge Iran to avoid any further steps away from JCPoA (Joint Comprehensive Plan of Action) and come back into compliance." — Agencies