Washington and its European allies suspect that Iran is developing the capability to produce atomic weapons under cover of a civilian nuclear energy program. Iran denies wanting atom bombs and insists its program is for generating electricity.

The report by the UN International Atomic Energy Agency (IAEA) is expected to unveil detailed intelligence pointing to military dimensions of Iran's nuclear program, while stopping short of saying explicitly that Tehran is trying to build such weapons.

"One (issue) in particular that I want to mention is the continuing threat posed by Iran's nuclear program," US President Barack Obama told reporters ahead of a G20 heads of state summit in the French resort of Cannes.

"The IAEA is scheduled to release a report on Iran's nuclear program next week and (French) President (Nicolas) Sarkozy and I agree on the need to maintain the unprecedented pressure on Iran to meet its obligations," Obama said.

Responding to a newspaper report that Britain was stepping up military contingency plans amid mounting concerns about Iran, a spokesman for the British Foreign Office said on November 2 that London was keeping all options open - including the possibility of military action.

The United States, the European Union and their allies around the world have imposed economic sanctions on Tehran for refusing to halt its uranium enrichment program, which Western powers believe is at the heart of an Iran atom bomb program.