The Taliban has vowed to seek revenge against “American savages” after a U.S. Army sergeant apparently walked into the homes of sleeping Afghans Sunday and killed them. Many of the victims were women and children. Civilian casualties have long been a major cause of friction between Washington and Kabul.

The U.S. Embassy has warned American citizens in Afghanistan about the possibility of reprisals

Afghan President Hamid Karzai has demanded an explanation for the killings. He said nine children and three women were among the victims. He said the attack was “an intentional killing of innocent civilians that cannot be forgiven.”

U.S. President Barack Obama called the Afghan leader to extend his condolences to the Afghan people and to express his “shock and sadness.” Mr. Obama said he stands behind Defense Secretary Leon Panetta's call for a quick investigation and holding accountable anyone who was responsible.

NATO officials say the suspect, identified as U.S. Army staff sergeant from a unit based in Washington state, turned himself in following the early morning shooting rampage in villages near a military base. Many witnesses have said there were several attackers, but officials have mentioned only one suspect.

The shootings in southern Afghanistan are likely to further fray Afghan-U.S. ties, already unraveling in the wake of last month's Quran burning on a U.S. military base in Afghanistan. Afghan forces killed six U.S. troops following the burning of the Muslim holy book as deadly protests erupted nationwide.

An ABC-Washington Post poll says 60 percent of U.S. citizens believe the war in Afghanistan is not worth its costs. The poll indicates 54 percent of all Americans want the U.S. to pull its troops from Afghanistan, even if the Afghan army is not adequately trained to carry on the fight.

In northern Afghanistan, German Chancellor Angela Merkel made an unannounced trip to visit German soldiers serving in the NATO-led security force. She arrived in Mazar-e-Sharif on Monday.