A Taliban suicide bomber in Kabul killed eight military officers Wednesday in an attack on an air force bus, Afghan officials said, in the latest strike against the national security forces as US troops withdraw.
The blast came the day after presidential election results were delayed in a worsening dispute over alleged fraud that threatens to tip Afghanistan into a prolonged period of uncertainty.
The next president must tackle the Taliban insurgency with declining NATO military assistance, and a political stalemate could wreck Afghan hopes for a smooth handover of power.
Kabul has been relatively peaceful since the election on June 14, though supporters of presidential candidate Abdullah Abdullah have held demonstrations to protest against alleged ballot-box stuffing.
"As a result of a suicide attack this morning on an ANA (Afghan National Army) air force bus in (west) Kabul, eight army officers were martyred and 13 wounded," defence ministry spokesman General Zahir Azimi said in a statement.
"The wounded personnel have been taken to hospital."
Sediq Sediqqi, spokesman for the interior ministry, told AFP that five civilians were also wounded in the attack that targeted the bus as it took military staff to work.
The Taliban used a recognised Twitter account to claim responsibility for the blast, adding that the suicide attacker approached the vehicle on foot before detonating his explosives-packed vest.
Taliban spokesman Zabiullah Mujahid said that at least 25 people had been killed, though the insurgents regularly exaggerate death tolls after attacks.
Election day saw a sharp rise in nationwide violence, but there has not been a major attack in the capital since June 7, when 12 people were killed in a suicide strike against Abdullah, who escaped unharmed.
Testing times for Afghan security
Abdullah has said he will reject the result of the on-going election vote count, alleging massive fraud in the race against his rival Ashraf Ghani.
Abdullah welcomed the delay in results as a chance for an "anti-fraud" audit of votes, although outgoing President Hamid Karzai has stressed the handover will stay on schedule with the new leader inaugurated on August 2.
US Secretary of State John Kerry called Karzai on Tuesday to urge him to address the political impasse.
Kerry also encouraged the election commission "to conduct a full and thorough review that ensures the Afghan people have confidence in the integrity of the electoral process," said a statement released in Washington.
NATO's 50,000-strong combat force will depart Afghanistan by December, though about 10,000 US troops may stay into next year if the new president signs a security deal with Washington.
Recent weeks have seen fierce fighting in the southern province of Helmand, with the Afghan army and police counter-attacking after a major offensive by 800 Taliban fighters focused on the strategic district of Sangin.
Sangin, a centre of Afghanistan's lucrative opium trade, has been the scene of bloody battles for years between the Taliban and US-led NATO forces, who only pulled out of the area in May.
With the NATO combat mission winding down, the coming months are expected to be a test of the Afghan government forces now responsible for security.
Amid the dispute over alleged fraud, the United Nations has warned of rising ethnic tensions and the threat of a "spiral of instability".