Iran has started cutting back on nuclear programmes, which could have been re-engineered into making weapons, in line with a deal between six world powers, the UN nuclear agency says.

The assessment was in a report presented by the UN's International Atomic Energy Agency (IAEA) to members of the UN Security Council and a restricted number of other nations.

Implementation day for the July 14 nuclear deal was October 18.

Since then, the report shows that Iran has reduced the number of centrifuges it uses to enrich uranium, a potential pathway to nuclear arms.

The report said 11,308 centrifuges were standing at Iran's main enrichment centre as of November 15.

That is more than 3,000 fewer than before the implementation day. It also noted reductions at a smaller facility.

However, diplomats familiar with the report cautioned that the country is keeping thousands of machines on standby.

The diplomats said all of the machines that have been taken out were idle. The thousands of centrifuges that were spinning uranium into enriched levels used for fuel are no longer online but remain on standby and can be restarted at short notice.