Published: Mar 12, 2012 11:19 AM EDT

KABUL, Afghanistan (AP) - The Taliban vowed revenge Monday after an American soldier allegedly shot to death 16 civilians in southern Afghanistan and burned their bodies.

The militant group promised the families of the victims that it would take revenge "for every single martyr with the help of Allah."

U.S.-led forces in Afghanistan have stepped up security following the shootings Sunday in Kandahar province out of concern about retaliatory attacks. The U.S. Embassy has also warned American citizens in Afghanistan about the possibility of reprisals.
The Taliban claimed responsibility for several attacks last month that the group said were retaliation for the Americans burning Qurans. Afghan forces also turned their guns on their supposed allies at the time, killing six U.S. troops as violent protests wracked the country.
 Afghan President Hamid Karzai condemned the attack Sunday and said the 16 dead included nine children and three women. Five other villagers were wounded.
"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Karzai said.
U.S. military spokesman Lt. Col. Jimmie Cummings confirmed Monday that the number of dead was "in the teens" but declined to be more specific, saying U.S. forces had not been given access to independently count the bodies.
U.S. officials said the shooter, identified as an Army staff sergeant, acted alone after leaving his base in southern Afghanistan. Initial reports indicated he returned to the base after the shooting and turned himself in. He was in custody at a NATO base in Afghanistan.
Some Afghan officials and local villagers expressed doubt that a single U.S. soldier could have carried out all the killings in houses about a mile apart and burned the bodies afterward.
Cummings, the U.S. military spokesman, also said, "There's no indication that there was more than one shooter."
The Afghan defense ministry said Monday that its initial reports indicate one soldier carried out the attacks, but they left open the possibility there could have been be more.
The soldier suspected of carrying out the attack is from Joint Base Lewis-McChord, Wash., and was assigned to support a special operations unit of either Green Berets or Navy SEALs engaged in a village stability operation, said a U.S. official, speaking on condition of anonymity because the investigation is still ongoing.
President Obama phoned Karzai on Sunday to express his shock and sadness, and offered condolences to the grieving families and to the people of Afghanistan.
In a statement released by the White House, The President  called the attack "tragic and shocking" and not representative of "the exceptional character of our military and the respect that the United States has for the people of Afghanistan." He vowed "to get the facts as quickly as possible and to hold accountable anyone responsible."