A suicide bomber killed eight people and wounded 13 aboard a bus carrying military personnel in Kabul. (Reuters)

A suicide bomber attacked an Afghan military bus in the capital on Wednesday, killing eight members of the country’s fledgling air force, authorities reported.

The Islamist Taliban insurgency said it was responsibile for the attack, which came amid growing uncertainty over a politi al impasse resulting from Afghanistan’s two-stage presidential election.

Afghan officials said the explosion occurred as the suicide bomber tried to force his way inside the bus, which was stopped at a usual pickup point for troops near Kabul University.

One civilian also was killed in the blast. About 20 others, many of them military personnel inside the bus, were wounded, officials said. It was the first such attack since suicide bombers struck the motorcade of presidential candidate Abdullah Abdullah in Kabul last month, killing seven people. Abdullah escaped unhurt.

With U.S. and NATO combat troops preparing to withdraw from Afghanistan by the end of the year, the Taliban has regularly been targeting Afghan security forces. But Wednesday’s attack was the deadliest against the Afghan air force, which faces serious shortages of equipment and training.

Meanwhile, Afghanistan’s political deadlock remains unresolved after the second round of presidential voting, held June 14, to determine the successor to Hamid Karzai: Abdullah, a former foreign minister, or Ashraf Ghani, a former finance minister and World Bank official.

Abdullah won the first round on April 5 by a wide margin but did not attain a majority, forcing the runoff. Unofficial results from the second round show Ghani well ahead. Those returns have prompted Abdullah to accuse the government-appointed Independent Election Commission (IEC) of massive fraud in favor of Ghani and to stop dealing with the commission, even though final results have yet to be released.

Thousands of Abdullah’s supporters have staged protests in Kabul and in other parts of the country in recent days. Some of them proclaimed him the winner of the election, while others have warned of violence if Ghani replaces Karzai.

Initial results, scheduled for release Wednesday, were delayed to allow a reassessment of ballots at 1,930 polling sites, one of Abdullah’s demands.

United Nations officials and First Vice President Yonus Qanooni, a key supporter of Abdullah, are mediating to bring Abdullah back into the process and to allow the first peaceful transfer of power through the ballot in Afghan history after decades of war.

Secretary of State John F. Kerry, who mediated between Karzai and Abdullah in a dispute over the 2009 presidential election, phoned Karzai on Tuesday and urged him to use his influence to end the impasse.

Final results are due to be released July 22, and Karzai has said repeatedly in recent days that he would hand over power to the next president on Aug. 2 as scheduled.

Abdullah and Ghani have promised to sign a security pact with the United States that would allow nearly 10,000 U.S. forces to remain in the country for two more years after 2014, when the mandate of Western troops expires in Afghanistan.