There had been suggestions that some in the US favoured a deal because of the challenges of holding a second vote. But both candidates told American media they were committed to another poll.

After a series of high-level diplomatic interventions - including phone calls and personal visits to Kabul - Mr Karzai agreed on Tuesday to a second round following August's fraud-marred election.

The UN-backed Electoral Complaints Commission (ECC) deducted hundreds of thousands of votes from the main candidates, pushing Mr Karzai beneath the 50 percent threshold needed for outright victory.

The panel also recommended replacing thousands of corrupt officials, and scrapping polling stations where the fraud was worst.

But, as campaigning officially began in Afghanistan, the Taliban threatened to launch a fresh wave of violence, urging people not to vote in what they called an "American process".