A wounded US soldier is carried away from the site of a suicide attack in Kabul October 29, 2011. [Photo/Agencies]

KABUL, Afghanistan — A Taliban suicide bomber rammed a van into an armored NATO bus Saturday in Kabul, killing 13 American troops and four Afghans, US and Afghan officials said, in the deadliest attack on coalition forces in more than two months.

The explosion, which occurred as the convoy was passing the American University, sparked a fireball and littered the street with shrapnel. Heavy black smoke poured from burning wreckage at the site.

The armored personnel carrier, known as a Rhino was sandwiched between of a convoy of mine-resistant military vehicles traveling on a four-lane highway frequently used by NATO forces in a southwestern section of the city.

NATO said 13 service members were killed, but a US official confirmed they were all Americans. The official spoke on condition of anonymity because of the sensitivity of the issue.

The Afghan Ministry of Interior said three Afghan civilians and one policeman also died in the attack. Eight other Afghans, including two children and four other civilians, were wounded, said Kabir Amiri, head of Kabul hospitals.

The Taliban claimed responsibility for the Kabul attack, as well as for another suicide bombing outside a government intelligence office in the northwest province of Kunar.

The attack occurred near the entrance of the American University and the nearby landmark Darulaman Palace, the bombed-out seat of former Afghan kings.

A NATO helicopter flies above the site of a bomb blast in Kabul October 29, 2011. [Photo/Agencies]

NATO and Afghan forces sealed off the area as fire trucks and ambulances rushed in. An Associated Press reporter at the scene saw two NATO helicopters landing to airlift casualties, while coalition troops using loudspeakers ordered bystanders to evacuate the area.

It was the deadliest single attack against the US-led coalition since the Taliban shot down a NATO helicopter on Aug 6 in an eastern Afghan province, killing 30 US troops, most elite Navy SEALs, and eight Afghans.

In other violence, a man wearing an Afghan military uniform opened fire on a joint NATO-Afghan base, killing three NATO service members in Uruzgan province, an area in the restive south that is traditionally viewed as the Taliban's stronghold.

Afghan Defense Ministry spokesman Mohammad Zahir Azimi said officials were investigating whether the shooter, who was killed in the incident, was a member of the Afghan army or a militant wearing an army uniform.