More than 70 people were injured in the attack
The bombing which killed at least seven people, three of them US soldiers, in north-west Pakistan on Wednesday was a suicide attack, police say.
Pakistani authorities earlier said the blast was caused by a remotely-detonated roadside bomb.
The physical evidence at the blast site appeared to support such a theory.
Correspondents say it is still unclear how the bomber knew that US soldiers were part of the convoy, as only the Pakistani military had those details.
But the authorities have not confirmed whether the soldiers were specifically targeted.
As well as the US soldiers, at least three schoolgirls and a Pakistani soldier were killed in Wednesday's attack, which happened close to a girls' school in the Lower Dir area of Pakistan's North West Frontier Province.
Some 70 others were injured.
A deep crater on the side of the road at the blast site and the absence of any body parts seemed to indicate the explosion was not caused by a suicide bomber, local journalists who visited the scene shortly after the attack said.
A spokesman for the Taliban in Pakistan told the BBC the group was responsible for the bombing.
"We are taking revenge for the US drone attacks in the tribal regions", Mohammad Umar, the spokesman said.
The spokesman also rejected recent reports that Pakistani Taliban leader Hakimullah Mehsud had been killed.
The soldiers were the first US military personnel involved in training in Pakistan's tribal areas to be killed.
But they are not the first US military casualties in Pakistan.
At least two US military personnel were killed when militants bombed the Islamabad Marriott in September 2008.
And in January 2002, seven soldiers were killed when a plane crashed into a mountainous area of western Pakistan.
The soldiers were among at least 60 to 100 members of a team that trains Pakistan's paramilitary Frontier Corps in counter-insurgency techniques, the New York Times reported on Wednesday.
Pakistan's Frontier Corps is a paramilitary force responsible for operations against militants in the volatile region near the Afghan border.
The BBC's M Ilyas Khan, in Islamabad, says the presence of the US military in Pakistan is a sensitive issue both in terms of security and in terms of public opinion.
Collaboration between the US and Pakistani armies would not be widely publicised in order not to inflame anti-American sentiment or sensitivities about Pakistan's military independence.
Both governments deny large numbers of US troops are in Pakistan.