The timing of Barack Obama’s much-awaited decision on how many new troops to send to Afghanistan has been thrown into further doubt amid mounting global speculation that Hamid Karzai, the Afghan president, may face a second round election run-off.

White House officials said on Friday that no decision would be possible on troop numbers or on the broader Afghan strategy until the political uncertainty had been resolved. It could take weeks to create a legitimate government in Kabul. Some analysts believe it would be logistically impossible to hold a second round of an election during the winter in Afghanistan.