In a document released by IAEA on Thursday, the United Nations watchdog said it was particularly worried about Tehran's potential to develop weapons, and chastised the country for not being totally honest with the inquiry into its nuclear capabilities.

"The agency remains concerned about the possible existence in Iran of undisclosed nuclear-related activities involving military-related organizations, including activities related to the development of a nuclear payload for a missile," it said.

For the past 18 months the Islamic republic has been meeting with Britain, China, France, Germany, Russia and the US—the so-called "P5+1" group. The two sides are attempting to strike a deal curbing Tehran's nuclear program, in exchange for tough international sanctions being lifted.

Iran has always denied it has nuclear ambitions.

An interim deal in late 2013 saw the nation's stockpile of fissile material diluted and some sanctions eased.

The P5+1 group is pushing for limits to be set on Iran's uranium enrichment capacity and for the country to open itself up to regular IAEA inspections.

Iran's deputy foreign minister, Abbas Araghchi, said on Thursday that diplomats from Iran and the United States would begin a new round of talks on Friday, which would then be continued by Iran's foreign minister, Mohammad Javad Zarif, and US Secretary of State John Kerry.

He said that senior envoys from the other five powers could join in later. The US State Department said the consultations "will take place in the context of the P5+1 nuclear negotiations with Iran."

Unanswered questions

Iran's deputy foreign minister, Abbas Araghchi, is expected to travel to Vienna next week to meet with the IAEA's director-general, Yukiya Amano, in a bid to move stalled talks forward.

The IAEA says Iran still has not answered questions on specifics, such as whether it has carried out research on nuclear arms.

Tehran has explained away suspicions, such as work related to detonators that could potentially be used to implode nuclear material in a warhead, as being designed for civil engineering and non-nuclear military purposes.

The IAEA is also trying to find out further information on high-explosives testing and computer modeling that could be related to developing nuclear weaponry.

However, the report did mention that, as part of agreed-to measures designed to build confidence, Iran has not increased uranium production, and has stopped constructing a plutonium reactor in the western city of Arak.

The leaders hope to have a deal outlined by the end of March, with the final details to be settled in the following months.

Both sides continue to disagree over the severity of the restrictions and when international sanctions would be lifted.

an/kms (dpa, AP, Reuters, AFP)