France, Germany and the United Kingdom started the process on Thursday to reimpose United Nations sanctions on Iran over its nuclear programme, after a 12-day conflict with Israel in June saw its atomic sites repeatedly bombed.

The mechanism, termed snapback by the diplomats who negotiated it into the 2015 nuclear deal (JCPOA) with world powers, was designed to be veto-proof at the UN and is likely to go into effect.

It would again freeze Iranian assets abroad, halt arms deals with Tehran and penalise any development of its ballistic missile program, among other measures, further squeezing the country's reeling economy.

The European move starts a 30-day clock for the sanctions to return, a period that could see intensified diplomacy from Iran, whose refusal to cooperate with the International Atomic Energy Agency's (IAEA) inspectors started the crisis.