NEW YORK, Aug. 21 (Reuters) - The United States was further isolated on Friday (Aug 22) over its bid to reimpose international sanctions on Iran with 13 countries on the 15-member UN Security Council expressing their opposition, 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 Britain, 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, seen by Reuters.

The United States has accused Iran of breaching a 2015 deal with world powers that aimed to stop Tehran developing nuclear weapons in return for sanctions relief.

But 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 United States 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.

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 U.N. sanctions should be reimposed at midnight GMT on Sep 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.