UN inspectors failed to reach a deal on Iran's atomic activity during talks in Tehran this week.
Herman Nackaerts, deputy director general of the International Atomic Energy Agency (IAEA), said his team had not been granted the access it has long sought to a military site.
A further meeting was scheduled for February 12.
In a separate note sent to IAEA member states about the negotiations, the UN agency said that important differences between the two sides remained.
The absence of an accord, meant to allay international concerns over Tehran's atomic ambitions, will disappoint world powers seeking a broader diplomatic settlement with Iran that would avert the threat of a new Middle East war.
The IAEA's efforts to unblock its stalled investigation into suspected atom bomb research in Iran are separate, but closely linked, to the negotiations between Tehran and six world powers, including the UK, that may resume later this month.
Speaking in Vienna ,Mr Nackaerts said: "We had two days of intensive discussions."
"We could not finalise the structured approach to resolve the outstanding issues regarding possible military dimensions of Iran's nuclear programme."
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules hereComments are closed on this article