The head of the UN nuclear watchdog is to visit Tehran this week as a December deadline looms for completion of a long-running investigation into its past activities, Iran's nuclear chief said.

A 12-strong team from the International Atomic Energy Agency was already headed to Iran and would be followed by director Yukiya Amano "in three or four days," Ali Akbar Salehi told Iranian television from Vienna late Monday.

An IAEA spokesman confirmed that a visit by Amano was under consideration but said details were still being worked out.

Salehi's announcement came after he held talks with Amano at IAEA headquarters in the Austrian capital.

"We discussed past and present nuclear activities in the country," he said.

The UN watchdog will be responsible for overseeing implementation of a landmark nuclear agreement reached between Iran and six world powers in July under which it is to rein in its nuclear programme in return for the gradual lifting of international sanctions.

But the IAEA is also separately tasked with concluding its decade-old investigation into allegations that at least until 2003, Iran's nuclear programme had "possible military dimensions" -- in other words that it conducted research into how to make an atomic weapon.

Iran has said repeatedly that those allegations are groundless and based on malicious intelligence provided by its enemies, but has agreed to help clear them up.

It signed an agreement with the IAEA on the same day that it signed the nuclear deal with the six powers setting out a roadmap for wrapping up the investigation by December 15.

Iran handed over a package of new information on August 15 but the IAEA said last week that it contained "ambiguities" that would need to be cleared up.

Salehi said Iran was eager to cooperate.

"We are following this matter seriously and we hope that in accordance with our agreement with the IAEA, we will achieve the target, so that on December 15 the issue will be closed... so that the nuclear deal can become operational," he said.

IAEA spokesman Fredrik Dahl confirmed only that talks were under way with Iran on implementing the July agreement, including the possibility of a visit by Amano.

"We are currently engaged in discussions with Iran on the implementation of the roadmap. In this context, a visit by the director general is also considered. Details are still to be worked out," he said.

Unlike the deal with the six powers, the agreement with the IAEA remains confidential, something that Iran's opponents in the US Congress have seized on in their campaign against both agreements.

Some Republican lawmakers have threatened legal action against the US administration, charging that its failure to disclose the IAEA agreement to Congress was a violation of its constitutional obligations.