Tortuous talks toward a Iran nuclear deal ploughed on Friday with the head of the UN's atomic watchdog having apparently failed in Tehran to advance a nuclear bomb probe, a major hurdle to the accord. Iranian President Hassan Rouhani's chief of staff, Muhammad Nahavandian, meanwhile headed to the negotiations in Vienna, for what the official IRNA news agency called a "special mission."

"Important progress has been made but questions on technical issues and the wording (of the deal) remain," Iranian Foreign Minister Muhammad Javad Zarif said on a seventh day of negotiations.

"My impression is that the political will (to get a deal) exists but that this has not yet been transmitted to the bureaucrats" working on the text, Zarif told Iranian television.

Ahead of a Tuesday deadline, the chief negotiators of Iran, the US and the EU haggled for six hours on Thursday night until 3:00 am (0100 GMT), a US official said.

"We have five days remaining ... The technical work is advancing on the main text, on the appendices," a diplomat said. "It feels like the end."

Other foreign ministers besides Zarif and Kerry were expected back in Vienna Sunday evening and to stay until Tuesday to get the job done, the diplomat said.

The P5+1 - the US, Russia, China, Britain, France and Germany - want an accord that curbs Iran's nuclear activities so that it making an atomic bomb is all but impossible.

In exchange Iran, which says its program is for peaceful purposes like electricity generation and not to get the bomb, would see painful sanctions progressively lifted.

It would end a 13-year standoff over Iran's suspected nuclear program, and draw the curtain on almost two years of intense negotiations.

Russia's top negotiator Sergei Ryabkov on Thursday voiced cautious optimism, saying the document was "91 percent" finished.

It will be up to the International Atomic Energy Agency (IAEA) to verify Iran is sticking to its side of the bargain through enhanced inspections of Iran's nuclear facilities.

On Thursday the IAEA chief Yukiya Amano visited Tehran to meet Rouhani and others in an attempt to jumpstart a stalled probe into these so-called "possible military dimensions" of Iran's activities.

But after returning a statement suggested that no breakthrough on the issue - which Western powers say is vital for the final deal - had happened.