A Taliban suicide car bomber attacked a NATO-led military convoy during rush hour in the Afghan capital on Tuesday, killing 12 Afghan civilians and six foreign troops, including five Americans, officials said.
A Taliban spokesman, claiming responsibility, told Reuters the Islamists had used a van packed with 750 kg of explosives.
The attack was the deadliest strike against foreign troops in the heavily guarded capital since September 2009, when six Italian soldiers were killed by a car bomb.
It comes after the Taliban announced a spring offensive against the Afghan government, foreign forces and diplomats in Afghanistan in response to NATO plans for an offensive on the group's southern stronghold of Kandahar.
"This will not deter us from our mission of securing a better future for this country," NATO spokesman Brigadier General Josef Blotz said in a statement.
The interior ministry said at least 12 Afghan civilians had been killed and 47 wounded. Most of the casualties were people waiting for a bus on the busy road near an army base, a government ministry and the parliament.
Kabul's Estiqlal hospital was overwhelmed with wounded people, including children, with their heads, legs and hands covered in blood. Some moaned in pain.
"The blast knocked me down, although I wasn't very near the explosion but I saw a van exploded and there was blood and bodies everywhere," said a teenager named Mustafa, whose head was wrapped in a blood-stained bandage.
Foreign troops were seen zipping up at least five of the dead in body bags. At least seven cars and one bus were seen destroyed in the attack. One SUV, a vehicle sometimes used by NATO forces, was completely burned amid the wreckage.
Police cordoned off the road near Darulaman palace, a derelict building that once housed Afghanistan's royal family, state television showed.
Afghan troops were collecting evidence and debris from the blast site.
Another survivor, government worker Noor Mohammad, was waiting for a bus when the bomber detonated his vehicle.
"A van driving very fast approached the convoy of foreigners and a huge blast went off ... I didn't know I was hurt, the explosion deafened my ears and I had a blackout," Mohammad, who suffered shrapnel wounds to his legs, said from his hospital bed.
President Hamid Karzai was holding a news conference at the time of the blast, following a trip to Washington where he met President Barack Obama to discuss strained ties between the two countries amid a rising insurgency and civilian casualties.
"I condemn this attack on strongest terms and hope that Afghanistan one day gets rid of this," Karzai said.
In the southeast, Afghan police shot dead a would-be suicide bomber who tried to attack a government building in Paktia province close to the Pakistani border, police said.
The bomber's explosives were detonated by the shooting, killing one policeman and wounding another.