The International Atomic Energy Agency will again highlight concerns over Iran's nuclear activities in its latest quarterly report, which is expected to be submitted to member states in the next few days.
"I expect it will be a bit tougher than the last one. Still a number of outstanding matters related to PMD (possible military dimensions) that Iran refuses to answer," a Western envoy told Reuters on Thursday.
An Iranian effort to show rare openness about its disputed nuclear program is doing little to dispel Western suspicions about Tehran's atomic ambitions, with one Vienna-based envoy dismissing it as just a "charm offensive".
Another diplomat painted a similar picture, saying Tehran had failed to address the IAEA's core concerns.
Western nations suspect Iran is trying to use its nuclear program to develop atomic weapons. The Islamic Republic has denied the charge, saying it wants to produce nuclear energy.
The UN International Atomic Energy Agency (IAEA) -- tasked with ensuring that nuclear technology is not diverted for military aims -- has repeatedly complained about Iran's lack of cooperation over allegations of military-linked nuclear work.
In previous reports, the IAEA complained Tehran refused to provide prompt access to sites, equipment, documents and people relevant for its probe. It also reported Iran had sought nuclear technologies unique to "implosion devices" - nuclear weapons more advanced than the bombs used on Hiroshima and Nagasaki.
In a move that Iran said showed the country's "100 percent transparency and openness," it allowed a senior IAEA inspector to tour the Islamic state's main atomic facilities last month, including one for developing advanced enrichment machines.
The IAEA has been trying since 2008 to gain access to sites linked to the manufacture of centrifuges used to refine uranium -- material which can have both civilian and military purposes -- but Iran had until now ignored the requests.
But the Western envoy suggested Iran was merely using an old tactic to ward off any harsher international pressure on the country, while pressing ahead with its nuclear work.
"The Iranians' recent charm offensive has not changed the Agency's view on what Iran still needs to do," he said.