Taliban officials also warned that journalists, aid workers and anyone considered to be assisting United States-led forces in Afghanistan would be targets for future attacks.
Referring to Sunday's killing of 29-year-old United Nations refugee agency official Bettina Goislard in Ghazni, southwest of Kabul, Taliban spokesman Mullah Abd al-Samad on Tuesday said:
"Yes we did that. Our guerrillas were involved in killing that Christian woman. We have confirmed information that most of the foreigners working in our country are American agents and have no sympathy for Afghanistan.”
"We will not spare them. They are not doing anything for common Afghans but are preaching Christianity in Afghanistan or spying against the Taliban," he said.
Goislard, shot by two men on a motorbike, was the first United Nations international staff member killed since the overthrow of the Taliban by US-led forces two years ago.
Afghan officials have already said they suspected the Taliban carried out the attack.
"They are not doing anything for common Afghans but are preaching Christianity in Afghanistan or spying against the Taliban"
Mullah Abd al-Samad
Late in October, the Taliban also kidnapped Turkish engineer Hassan Onal, who was working on a road project in the south.
Mullah Sabir, alias Momin, a senior Taliban commander for southern Afghanistan, said they wanted the release of comrades in jail in Ghazni in return for his freedom.
"But the Afghan administration is not showing any interest in his release. If the situation persists, we will be forced to kill him and his body will soon be found," Sabir said.
Sabir added that anyone working in the interests of the US was liable to be killed. "This includes journalists, NGO activists, drivers, engineers, and others."
Sabir said they were regrouping in the southern province of Zabul, which they had chosen as their headquarters.
He said that on the instructions of Taliban supreme leader Mullah Muhammad Omar, Taliban members living outside Afghanistan had been ordered back and "hundreds" were reaching Zabul daily.
"Thousands of Taliban, equipped with arms and other war weapons, are present in Zabul and are ready for war," Sabir added.
Sabir said the Taliban controlled up to seven districts of Zabul and planned to intensify attacks on US-led forces despite the onset of winter.
A Zabul tribal leader, Muhammad Khan, said armed Taliban were able to roam freely in several districts, "and officials loyal to (President) Hamid Karzai are confined to government offices".