Half of the top Afghan district election officials will be fired, UN officials said yesterday as they sought to prevent more fraud in a run-off Presidential poll crucial to the government's credibility and foreign support.

The announcement of a November 7 run-off on Tuesday had removed one stumbling block for US President Barack Obama as he weighed whether to send more troops to Afghanistan to fight a resurgent Taliban.

In Washington, the White House said it was possible Mr Obama could make a decision on troop levels before the run-off, but there was no guarantee.

Amid flagging US public support for the war, the Obama administration has stressed a credible government must be in place in Kabul for any US strategy there to succeed.

"The President and the entire team see legitimacy of a partner in the government of Afghanistan as crucial," said spokesman Robert Gibbs.

Afghan President Hamid Karzai's decision to accept the run-off against his main rival, Abdullah Abdullah, came after days of diplomatic wrangling and helped ease tensions with the West.

Concerns about a repeat of the widespread fraud that tainted the first round in August cast a long shadow as hasty preparations for the second round kicked off.

Mr Karzai agreed to face a second round after a UN-led fraud inquiry tossed out enough of his first-round votes to push him below 50 per cent and trigger a run-off.

The United Nations mission in Afghanistan, which provides assistance with election-related operations, said preparations were under way to block any risk of mass fraud.