This starts the clock on negotiating a trickier long-term accord aimed at ending the Iran nuclear standoff and averting war once and for all, a process threatened however by possible new US sanctions.

Iran's Foreign Minister Mohammad Javad Zarif on Sunday said he hoped for "positive results for the country, as well as regional and global peace and security".

UN inspectors from the International Atomic Energy Agency arrived in Tehran on Saturday standing ready to report back first thing from sites around Iran that the freeze has begun.

On day one, Iran has to halt the enrichment of uranium to medium levels, close to weapons-grade, and to begin diluting half of its stockpile of this material.

If the IAEA gives the thumbs-up, in Brussels EU foreign ministers will adopt legislation loosening sanctions on items like auto parts and gold, followed later in the day by a similar move in Washington.

Over the next six months Iran will also not install or switch on new nuclear machines and will grant the IAEA more access, including daily visits to the Fordo and Natanz enrichment facilities.

The total sanctions relief, staggered over the six months, is worth some USD 6-7 billion, including USD 4.2 billion in frozen overseas assets. The first USD 550 million installment is due February 1.

Over the next half-year alone, Iran will miss out on USD 30 billion in oil revenues, the White House says. Most of Iran's USD 100 billion in foreign exchange holdings remains off-limits.

Iran and the P5+1 countries, US, China, Russia, Britain, France and Germany, will soon begin talks on a long-term comprehensive accord.

According to Mark Fitzpatrick, a former US State Department official now at the International Institute for Strategic Studies, the powers will want Iran to slash the number of centrifuges to 3,000-4,000 from the current 19,000.

(Agencies)

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