KABUL, Afghanistan -- A Taliban suicide bomber rammed a vehicle loaded with explosives into an armored NATO bus Saturday on a busy thoroughfare in Kabul, killing 17 people, including a dozen Americans, in the deadliest strike against the U.S.-led coalition in the Afghan capital since the war began.
The blast occurred on the same day that a man wearing an Afghan army uniform killed three coalition troops, who were reportedly Australian, in the south -- attacks that are likely to raise new doubts about the unpopular 10-year-old war and the Western strategy of trying to talk peace with the Taliban.
A spokesman for the fundamentalist Islamic movement, which was ousted in the 2001 invasion for its affiliation with al-Qaida, claimed responsibility for the attack, saying the bomber had used 1,540 pounds of explosives.
The Taliban and related groups have staged more than a dozen major attacks in Kabul this year, including seven since June, in an apparent campaign to weaken confidence in the Afghan government as it prepares to take over its own security ahead of a 2014 deadline for the U.S. and other NATO countries to withdraw their troops or move them into support roles.
Underscoring the difficulties ahead, the brazen assault occurred just hours after top Afghan and Western officials met in the heart of Kabul to discuss the second phase of shifting security responsibilities to Afghan forces in all or part of 17 of the country's 34 provinces. Afghans already have the lead in the Afghan capital.
NATO said five of its service members and eight civilian contractors working for the coalition died in the attack.
A U.S. defense official, speaking on condition of anonymity to release the information before a formal announcement, said all 13 were Americans. However, Lt. Col. Christian Lemay, a Canadian defense spokesman, said that one Canadian soldier was among the troops killed. The discrepancy could not immediately be reconciled.
It was the deadliest single attack against the U.S.-led coalition across the country since the Taliban shot down a NATO helicopter on Aug. 6 in an eastern Afghan province, killing 30 U.S. troops, most elite Navy SEALs, and eight Afghans.
The Afghan Ministry of Interior said four Afghans, including two children, also died in Saturday's attack. Eight other Afghans, including two children, were wounded, said Kabir Amiri, head of Kabul hospitals.
Afghan Defense Ministry spokesman Mohammad Zahir Azimi said officials were investigating whether the man who opened fire on a joint NATO-Afghan base in the restive southern Uruzgan province was an actual soldier or a militant in disguise. NATO did not give the nationality of the three service members killed, but the Australian Broadcasting Corp. reported that they were Australian.
Just a day earlier, the Pentagon issued a progress report saying that the number of enemy-initiated attacks in Afghanistan was trending downward.
However, the Pentagon also noted that the insurgency's safe havens in Pakistan and the limited capacity of the Afghan government could jeopardize efforts to turn security gains on the battlefield into long-term stability in Afghanistan.