The diplomats and nuclear experts briefed on the findings by UN officials said a rouge Soviet scientist had tutored Iranians over several years on building high-precision detonators for the weapons.
The intelligence update would be circulated among members of the International Atomic Energy Agency (IAEA) this week and is expected to focus on Iran's alleged efforts towards putting radioactive material in a warhead and developing missiles.
Fears that Iran could quickly build an atomic bomb, if it choses, has fueled new threats of military strikes.
Israeli President Shimon Peres yesterday warned that an attack on Iran was "more and more likely," days before the report by the UN nuclear watchdog. He told Israeli private television's second channel: "The intelligence services of the different countries that are keeping an eye on (Iran) are worried and putting pressure on their leaders to warn that Iran is ready to obtain the nuclear weapon."
The key breakthrough in Iran's quest for nuclear weapons was its success in obtaining design information for a device known as R265 generator.