Iran is suspected of conducting secret experiments whose sole purpose is to develop nuclear arms, the UN nuclear atomic energy agency said for the first time.
The report is the strongest sign yet that Iran seeks to build a nuclear arsenal, despite claims to the contrary. With Israel threatening a military response, the report opens the way for a new confrontation between the West and Iran.
In its latest report on Iran, the International Atomic Energy Agency outlines the sum of its knowledge on the Islamic Republic's alleged secret nuclear weapons work, including clandestine procurement of equipment and design information needed to make such arms.
The report also highlighted high explosives testing and detonator development to set off a nuclear charge, computer modelling of a core of a nuclear warhead and preparatory work for a nuclear weapons test.
It also noted the development and mounting of a nuclear payload onto its Shahab 3 intermediate range missile - a weapon that can reach Israel, Iran's arch foe.
Israeli government spokesman Mark Regev said there is a government directive not to comment until Israel has studied the report in depth.
But ahead of the report's release, Israeli Defence Minister Ehud Barak warned of a possible Israeli military strike against Iran's nuclear programme.