By Hamid Shalizi
KABUL (Reuters) - 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.
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.
In a separate bomb attack, another member of the NATO-led International Security Assistance Force (ISAF) was killed in southern Afghanistan, ISAF said in statement.
It gave no other details. Tens of thousands of mostly U.S. and British troops are in the southern provinces of Helmand and Kandahar, both Taliban strongholds, trying to turn the tide of a growing insurgency.
In Ghazni province in the southeast, the Taliban beheaded three Afghan boys accused of being spies, a police official said.
In neighboring Paktia province of Paktia, close to the Pakistan border, Afghan police shot dead a would-be suicide bomber who tried to attack a government building, police said.
The shooting detonated the explosives, killing one policeman and wounding another.
President Hamid Karzai was holding a news conference at the time of the Kabul blast. Last week, he met U.S. President Barack Obama to discuss strained ties between the two countries amid a rising insurgency and civilian casualties.
He condemned the attacks, which came weeks before he hosts a grand assembly of Afghans to present his draft negotiations with the Taliban and to seek advice from delegates on peace moves as part of efforts to reconcile with the group.
He also said the West was starting to realize the war in Afghanistan cannot be won militarily and that the peace process must involve reconciling with the Taliban.
" ...it took us a lot of time to make them understand our intention ... that our intention is to bring peace to Afghanistan and that militarily this war cannot succeed," Karzai said.
The suicide attack overwhelmed Kabul's Estiqlal hospital 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 zipped at least five of the dead in body bags. At least seven cars and a bus were destroyed in the attack.
Police cordoned off the road near Darulaman palace, a derelict building that once housed Afghanistan's royal family and Afghan troops gathered 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.
(Additional reporting by Golnar Motevalli, Ahmad Masood and Sayed Salahuddin; Writing by Alistair Scrutton; Editing by Paul Tait)