Iran acknowledged on Friday that it still has differences with the UN atomic watchdog after two days of talks over its disputed nuclear activities, but also said that the negotiations are progressing.

"Some of the differences were solved but it is very complex issue... No agreement has been signed but the negotiations are progressing," Iran's International Atomic Energy Agency envoy Ali Asghar Soltanieh said on television.

"We hope that they take our considerations into the document; it is a good opportunity to reach an agreement," he said of a "structured approach" deal that would see Iran answer allegations of a military dimension to its nuclear drive.

The IAEA conducts regular inspections of Iran's declared nuclear facilities but also wants access to what it believes are sites where undeclared activities aimed at developing nuclear weapons took place until 2003, and possibly since.

Tehran denies working on or ever having worked on the bomb and says the IAEA's information is based on faulty foreign intelligence -- material that it has not been shown.

Soltanieh's comments came a day after a team led by IAEA chief inspector Herman Nackaerts wrapped up two days of crunch talks in Tehran -- the second round in a month.

"We decided to have more talks," Soltanieh said after what he called "serious and intensive negotiations."

"When we agree on everything, we will sign it and we let people know the content of the deal."

Nackaerts said in Vienna on Friday that "differences remain so we could not finalise the structured approach to resolve the outstanding issues regarding possible military dimensions of Iran's nuclear programme."

He did not elaborate.

The only agreement was for both sides to meet again in Tehran on February 12.

Soltanieh again dismissed the possibility of the IAEA inspecting the contested Parchin military site, however.

"As we said, until such time as we ink the agreement they would not have access to the sites they want to inspect," he said.

Iran argues that because no nuclear activities have taken place at Parchin, the IAEA has no business inspecting it.

It was unclear what went wrong this time, but in the past Iran has insisted that the agreement include clauses that could hamper the IAEA's ability to conduct proper inspections.

This included the IAEA agreeing to "close" an issue for good once it had been covered, even if new information came to light, or the agency being able to inspect a site or a document only once, diplomats said.

Soltanieh said delays in reaching a deal boil down to consenting on "terms and phrases."

"I have to say that in the past year of discussions we were talking about a two or three page document in which a single term or phrase takes hours to decide on because of political and security repercussions," he said.

Soltanieh said "the agency cannot enter domains which could infringe upon our national security," adding: "The talks are ongoing but we are ready to find ways to answer these ambiguities... our cooperation under the NPT (Non-proliferation Treaty) will continue."

The latest talks came ahead of a new meeting between Iran and the P5+1 powers -- the US, China, Russia, Britain, France and Germany.

It had been expected to take place this month but has not yet been arranged, either because Iran was waiting until after the IAEA talks or because there are problems agreeing on an agenda.

This parallel effort is focused more on Iran's current activities, in particular uranium enrichment, a process that can be used for peaceful purposes but also for creating the core of a nuclear bomb.

At their last meeting in Moscow in June, Tehran rejected P5+1 calls for it to scale back its enrichment activities, while asking for relief from sanctions that in 2012 began to bite.

Soltanieh told Iran's Arabic-language Al-Alam on Friday that "we will not stop enrichment for even a moment... it is our undeniable right to do so."