A Taliban suicide bomber rammed a vehicle loaded with explosives into an armoured Nato bus on a busy thoroughfare in Kabul, killing 17 people, including a dozen Americans, in the deadliest strike against the US-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 Australian soldiers and an Afghan interpreter in the south - attacks that show the resiliency of the insurgency and 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 Kabul attack, saying the bomber had used 1,540lbs (700 kilograms) 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 US 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.
Heavy black smoke poured from the burning wreckage of an armoured personnel carrier, known as a Rhino, in Kabul after the bomber struck. The bus had been sandwiched in the middle of a convoy of mine-resistant military vehicles when it was hit along a four-lane highway often used by foreign military trainers in the southwestern part of Kabul.
Nato said five of its service members and eight civilian contractors working for the coalition died in the attack.
A US defence official said all 13 were Americans. However, Lt Col Christian Lemay, a Canadian defence spokesman, said one Canadian soldier was among the troops killed. The discrepancy could not immediately be reconciled.
It was the deadliest single attack against the US-led coalition across the country since the Taliban shot down a Nato helicopter on August 6 in an eastern Afghan province, killing 30 US 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.