A Taleban suicide bomber has rammed a vehicle loaded with explosives into an armoured Nato bus on a busy road in Kabul, killing 17 people including a dozen Americans in the deadliest strike against the Western coalition in the Afghan capital since the war began.

A spokesman for the Taleban claimed responsibility for the attack, saying the bomber had used 700kg of explosives.

The Taleban 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 United States and other Nato countries to withdraw their troops or move them into support roles.

The assault occurred just hours after top Afghan and Western officials met in 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 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.

The Darulaman Palace, the bombed-out seat of former Afghan kings, was the backdrop to the chaotic scene – shrapnel, twisted pieces of metal and charred human remains littered the street.

US soldiers wept as they pulled bodies from the debris, said Noor Ahmad, a witness. One coalition soldier was choking inside the burned bus, he said. “The bottom half of his body was burned.”

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, Lieutenant Colonel Christian Lemay, a Canadian defence spokesman, said one Canadian soldier was among the troops killed.

The Afghan Ministry of Interior said four Afghans, including two children, also died in the attack.