The UN nuclear watchdog says Iran has stopped a team of inspectors from visiting a key military site.
The International Atomic Energy Agency (IAEA) says no deal was been reached on inspecting the Parchin site, south of Tehran, despite "intensive efforts".
The inspectors had sought to clarify the "possible military dimensions" of Iran's nuclear programme.
Iran says its programme is for peaceful purposes, but the West suspects it is geared towards making weapons.
The IAEA says its team is returning from Iran without a deal after two days of talks. The first round of discussions in January also failed to produce a result.
"It is disappointing that Iran did not accept our request to visit Parchin," IAEA Director General Yukiya Amano said in a statement.
It is suspected that Parchin was the site where explosives related to nuclear weapons may have been tested in recent years.
Tehran has made no public comments of the latest remarks by the Vienna-based IAEA.
The BBC's Bethany Bell in Vienna says the refusal to grant access to Parchin does not come as a major surprise, as there has been so little progress in the negotiations between Iran and the IAEA.
The inspectors' evaluation of their visits may form part of the next report on Iran's nuclear programme, expected later in February.
But last November, the IAEA said it had information suggesting Iran had carried out tests "relevant to the development of a nuclear explosive device".
This led to decisions by the US and the European Union to tighten sanctions against Tehran, including measures targeting the country's oil industry.
Tensions have risen further over speculation that Israel may carry out a military strike on Iran's nuclear facilities.----
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