"Deeply worried by Iran's announcement that it has broken existing nuclear deal obligations," Hunt, a candidate to become Britain's next prime minister, said on Twitter.

"UK remains committed to making deal work (and) using all diplomatic tools to deescalate regional tensions. I urge Iran to avoid any further steps away from JCPOA (nuclear deal and) come back into compliance," he added.

Foreign Minister Mohammad Javad Zarif said earlier Monday that Iran had crossed the limit, Iran's semi-official news agency ISNA reported, in a move seen as retaliation to new US sanctions.

The International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, confirmed the development a short time later.

The United States withdrew from the nuclear deal last year and reimposed biting sanctions on Iran's crucial oil exports and financial transactions as well as other sectors.

Tehran, which has sought to pressure the remaining parties to save the deal, on May 8 announced it would no longer respect the limit set on its enriched uranium and heavy water stockpiles.

It also threatened to go further and abandon more nuclear commitments unless the remaining partners -- Britain, China, France, Germany and Russia -- helped it to circumvent sanctions, especially to sell its oil.