Afghan presidential challenger Abdullah Abdullah looks poised to withdraw from next Saturday’s second-round ballot.He is reportedly on the verge of quitting after incumbent Hamid Karzai failed to bow to a series of demands. Among other things, Abdullah wants Afghanistan’s top election official to be sacked. The run-off was triggered by a UN-led investigation into the first-round of voting in August that found widespread fraud in favour of Karzai. Abdullah’s team fears that unless radical measures are taken, massive irregularities will mark the second round as well. But some observers think Abdullah’s demands were so unlikely to have been acceptable to Karzai that they amount to an exit strategy. As well as fears over fraud, security concerns also surround the poll which the Taliban has vowed to disrupt. Their threat was underlined by a suicide attack in Kabul on Wednesday in which five foreign UN staff were killed.