The Taliban vowed to avenge the deaths of 16 civilians killed in southern Afghanistan during a shooting spree perpetrated by an American soldier.

On Sunday morning, a U.S. Army staff sergeant believed to have acted alone entered two villages and went from house-to-house, shooting civilians, most of them women and children. The two villages were close to a coalition base in the Kandahar province.

NATO tightened security in Kandahar immediately after the shooting spree for fear the Taliban would take revenge on the sick-minded American savages, CNN reported. The incident occurred around 3 a.m.

The Islamic Emirate assures survivors of the incident that it will, with God Almighty's assistance, avenge the martyrdom of each of the victims of the invading and beast-like murderers and punish them for their barbaric action, the Taliban said in a statement.

Relations between Kabul and Washington are hanging by a thread. Sunday's killings came as the dust following the Koran burning incident and subsequent riots last month was only just beginning to settle.

The Afghan people can withstand a lot of pain, Prince Ali Seraj, the head of the National Coalition for Dialogue with the Tribes of Afghanistan, told CNN.

They can withstand collateral damage. They can withstand night raids. But murder is something that they totally abhor, and when that happens, they really want justice.

U.S. President Barack Obama called the incident tragic and shocking, and offered his condolences to the families and loved ones of those who lost their lives, and to the people of Afghanistan. But, as in the aftermath of the Koran burning, Obama's apology did little to temper the anger coming from as high up as Afghan President Hamid Karzai.

[The] Kandahar incident was an intentional massacre and is an unforgivable act, Karzai said.

So far, there have been no protests or attacks in Afghanistan following the killing spree. The soldier responsible has turned himself in to base officials.

We don't know what motivated this individual, and we're not sure where this is going to take us, Capt. John Kirby, spokesman for the International Security Assistance Force, told CNN.