It has refused to halt uranium enrichment activities despite three UN Security Council resolutions.

Iran submitted its package of proposals to the six countries as the head of the UN-backed nuclear watchdog, the International Atomic Energy Agency (IAEA), said there was a 'high probability' that Iran had tried to develop nuclear weapons in the past.

The US envoy to the IAEA, Glyn Davies, said Tehran may already have enough enriched uranium for one atomic bomb.

Crowley said the US remained open to direct talks with the Iranians. Discussions with the other five powers involved in the talks would in part be aimed at testing 'how ready Iran is to actually engage,' he said.

The last meeting between Iran and the so-called 5+1 group was held in July 2008 in Geneva.