The UN's 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 has been reached on inspecting Parchin, south of Tehran, despite "intensive efforts".
IAEA inspectors had sought to clarify the "possible military dimensions" of Iran's nuclear programme.
Iran insists it is purely for peaceful purposes, but the West suspects it is seeking to build a nuclear weapon.
The IAEA said that after two days of talks, its team was returning from Iran without a deal on a document "facilitating the clarification of unresolved issues".
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. "We engaged in a constructive spirit, but no agreement was reached."
In November, an IAEA report - based on what it called "credible" information - indicated that Iran had built in 2000 a large explosives containment vessel at Parchin to conduct hydrodynamic experiments.
Hydrodynamic experiments, which involve high explosives in conjunction with nuclear material or nuclear material surrogates, were "strong indicators of possible weapon development", the report said.
In addition, the use of surrogate material and the confinement provided by a chamber could be used to prevent contamination of the site with nuclear material, the report added.
Iran's ambassador to the IAEA, Ali Asghar Soltanieh, said at the time that there were no nuclear-related activities at Parchin.
Israeli strike speculation
On Tuesday, Mr Soltanieh said Tehran expected to hold further talks with the IAEA. He was quoted by Iran's Isna news agency as saying the latest discussions had been intensive and that talks would continue.
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 little progress in the negotiations between the two sides.
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.