The United States Tuesday said Iran's refusal to respond to queries from the UN atomic watchdog is "very troubling," warning its behaviour bolstered suspicions it aimed to build a nuclear bomb.

The US government, which has spearheaded toughening rounds of sanctions against Tehran, added it was studying what "diplomatic next step" to take after Modnay's report from the International Atomic Energy Agency (IAEA). France and Germany - which along with Britain,

Russia and China - locked in carrot-and-stick negotiations with Iran over its nuclear programme - voiced immediate concerns about the report.

The IAEA expressed "serious concern" on Monday that Iran is still hiding information about alleged studies into making nuclear warheads and defying UN demands to suspend uranium enrichment.

The alleged studies comprise a uranium conversion project, high explosives testing and designs for a missile re-entry vehicle, and suggest there may have been a possible military angle to past nuclear work.