Afghan presidential challenger Abdullah Abdullah quit an election run-off on Sunday after accusing the government of not meeting his demand for a fair vote, leaving doubts over the legitimacy of the next government.

A weakened Afghan government under President Hamid Karzai would also be a blow for U.S. President Barack Obama as he decides whether to send up to 40,000 more U.S. troops to fight a resurgent Taliban in Afghanistan.

Karzai's spokesman also ruled out a coalition with Abdullah, dashing hopes that it might have been a way out of the impasse.