Wednesday’s attack, which left six children and a woman among the dead, came on one of the bloodiest days since the Taliban government was ousted from power in late 2001.
More than 60 people died in the 24 hours from late Tuesday, 25 of them in a factional clash and 21 in fighting between Afghan forces and suspected Taliban and Al-Qaida rebels near the border with Pakistan.
"The Taliban are busy ousting infidels from Afghanistan and the Taliban are carrying out activities against their agents," Mullah Abd Al-Rauf, a Taliban official, told Reuters by satellite telephone from an undisclosed location.
Rauf blamed the attack on rival Afghan commanders settling old scores.
"The Taliban are not linked to the deaths of innocent people and the Taliban feel sorry for those deaths," he said, referring to the Helmand bus attack.
But Rauf also accused non-governmental organisations working in Afghanistan of acting as spies for the United States, adding that attacks on them would therefore be justified.
He did not specifically claim responsibility for the deaths on Wednesday of two Afghans working for the Afghan Red Crescent Society. Their vehicle was ambushed to the southwest of the capital and three members of the group were also injured.
The Taliban have been blamed for attacks on NGOs, anti-Taliban clerics, Afghan and foreign forces and civilians in recent months.