"One word: it's a lie. It needs no more explanation," foreign ministry spokesman Hamid Reza Asefi told reporters.

Diplomats and analysts at the headquarters of the International Atomic Energy Agency (IAEA) told AFP the US has briefed key nations on intelligence it says shows Iranian atomic weapons work.

The US allegations centre around information purported to show Iranian research on getting a missile warhead to explode at an altitude that would maximize the blast of a nuclear explosion.

Iran says its nuclear program is a strictly peaceful effort to develop atomic power in order to generate electricity and rejects US charges that it is secretly developing nuclear weapons.

The briefings ahead of an IAEA board meeting in September were part of campaigning for a resolution that found Iran in non-compliance with international nuclear safeguards and could lead to referring Tehran to the UN Security Council for possible sanctions.