An IAEA report verifying that Iran has kept its promises under last year's nuclear deal with world powers and triggering sanctions relief for Tehran is likely to be issued on Saturday, a diplomatic source said on Friday.

The report, if issued, would mark the consummation of the July 14, 2015 nuclear agreement. Under the deal, Iran agreed to shrink its atomic programme in exchange for the lifting of some EU, US and U.N sanctions, which would allow billions of dollars of investment to flow into the country.

In a sign its implementation may be at hand, US Secretary of State John Kerry, Iranian Foreign Minister Mohammad Javad Zarif and EU foreign policy chief Federica Mogherini will meet in Vienna on Saturday, the US State Department said.

"All parties have continued making steady progress towards Implementation Day of the JCPOA, which will ensure the exclusively peaceful nature of Iran's nuclear program," said State Department spokesman Mark Toner, referring to the formal title of the deal, the Joint Comprehensive Plan of Action.

Iranian and other officials had previously said they expected the report from the U.N. nuclear watchdog, the Vienna-based International Atomic Energy Agency, to come out on Friday.

"Almost all details are ironed out," said another diplomatic source, based in the Austrian capital.

The IAEA is in charge of verifying that Iran has carried out all of the nuclear-related steps required in the deal it struck with the United States, Russia, China, Britain, France and Germany. It must release a report once it has done so.

The IAEA declined comment on the timing of the report.

Iranian officials have said Zarif and Mogherini would issue a statement on Saturday or Sunday on the "Implementation Day" of the nuclear deal and the lifting of sanctions.

Since July, Iran has drastically reduced the number of centrifuges installed at its enrichment sites, shipped tonnes of low-enriched uranium materials to Russia and dismantled the core of its Arak nuclear reactor.

U.N. spokesman Stephane Dujarric said there could be a meeting in relation to Iran on Saturday in Vienna, where the July 14 deal was reached, but did not elaborate.

"There may be some sort of a meeting tomorrow in Vienna, after which, if everything goes well, we will issue a statement from the Secretary-General," he told reporters.

In another sign implementation may be near, US President Barack Obama delegated authority to Kerry to take steps to ease some sanctions.

However, a US official said this was "one of many preparatory steps" Washington had to take to ease sanctions once the IAEA verifies Iran has met its nuclear obligations. – Reuters