The so-called "Swiss Paper" proposes that Iran freeze its work on sensitive nuclear activities while world powers halt UN sanctions action, in the hope that this mutual pause will create room for manoeuvre.

Bolton, who was a leading hawk in the Bush administration in the run-up to the invasion of Iraq, told Swiss newspaper SonntagsZeitung that the plan is a "farce".

"Switzerland should keep out of it," he added.

Bolton stepped down as ambassador to the UN last December, after it became clear the new Democrat-led Congress would not confirm his appointment.

He now works for the American Enterprise Institute, a leading conservative think tank in Washington.

Iran earlier this month also rejected the proposals as unacceptable.

"We believe it is a kind of suspension to which we have given a negative response and will not accept," said Abdolreza Rahmani Fazli, the deputy secretary of Iran's Supreme National Security Council.

The United States accuses Iran of using its nuclear programme to secretly develop an atomic bomb. Tehran denies the charge and insists its nuclear activities are for peaceful purposes.

Enrichment is hugely sensitive because the process can be used to make the core of an atom bomb as well as nuclear fuel.

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