The United States was further isolated on Friday over its bid to reimpose international sanctions on Iran, with 13 countries on the 15-member United Nations Security Council expressing their opposition and arguing that Washington's move is void given it is using a process agreed under a nuclear deal that it quit two years ago.

In the 24 hours since US Secretary of State Mike Pompeo said he triggered a 30-day countdown to a return of UN sanctions on Iran - including an arms embargo - long-time allies the United Kingdom, France, Germany and Belgium as well as China, Russia, Vietnam, Niger, Saint Vincent and the Grenadines, South Africa, Indonesia, Estonia and Tunisia have already written letters in opposition, Reuters news agency reported.

The US has accused Iran of breaching a 2015 deal with world powers that aimed to stop Tehran from developing nuclear weapons in return for sanctions relief. But US President Donald Trump described it as the "worst deal ever" and quit in 2018.

Diplomats said Russia, China and many other countries are unlikely to reimpose the sanctions on Iran. Pompeo again warned Russia and China against that on Friday, threatening US action if they refuse to reimpose the UN measures on Iran.

The Trump administration on Friday dismissed the near universal opposition to its demand and declared that a 30-day countdown for the "snapback" of penalties had begun.

"We don’t need anyone’s permission," US special envoy for Iran Brian Hook told reporters in a briefing on Friday. "Iran is in violation of its voluntary nuclear commitments. The condition has been met to initiate snapback. And so we have now started to initiate snapback."

He said that "whether people support or oppose what we’re doing is not material," adding that "today is day one of the 30-day process."

The US acted on Thursday after the Security Council resoundingly rejected its bid last week to extend an arms embargo on Iran beyond its expiration in October. Only the Dominican Republic joined Washington in voting yes.

Iran's Ambassador to the UN Majid Takht Ravanchi immediately rejected the US move, which he said was "doomed to failure".

The Dominican Republic has not yet written to the council to state its position on the sanctions snapback push.

Under the process Washington says it has triggered, it appears all UN sanctions should be reimposed at midnight or 00:00 GMT (8pm New York time) on September 19 - just days before Trump is due to address world leaders at the UN General Assembly, the annual meeting that will be largely virtual because of the coronavirus pandemic.

What now?

A 2015 Security Council resolution enshrining the nuclear deal states that if no council member has put forward a draft resolution to extend sanctions relief on Iran within 10 days of a noncompliance complaint, then the body's president shall do so within the remaining 20 days.

The US would be able to veto this, giving it a cleaner argument that sanctions on Iran have to be reimposed.

However, the 2015 resolution also says the council would "take into account the views of the states involved". Given the strong opposition, some diplomats say the council president - Indonesia for August and Niger for September - would not have to put up a draft text.

"Faced with this very strong view of a majority of Security Council members that the snapback process has not been triggered, as the presidency they are not bound to introduce the draft resolution," UN Security Council diplomat told Reuters, speaking on condition of anonymity.

Pompeo and Hook signalled that Washington expects Indonesia or Niger to put a text to a vote. Another US option is to put forward the draft itself or ask the Dominican Republic to do so.

The US argues that it can trigger the sanctions snapback process because the 2015 Security Council resolution still names it as a nuclear deal participant.

However, in a joint letter to the Security Council on Thursday hours after the US submitted its complaint, the UK, Germany and France said: "Any decisions and actions which would be taken based on this procedure or on its possible outcome would also be devoid of any legal effect."

UN Secretary-General Antonio Guterres distanced himself from the showdown in the Security Council.

"Security Council members will need to interpret their own resolution," UN spokesman Stephane Dujarric told reporters. "It's not the Secretary-General."