Afghan officials say a U.S. service member shot and killed at least 16 Afghan civilians in Kandahar Sunday, renewing tensions between Washington and Kabul.

Afghan President Hamid Karzai called Sunday's shooting unforgivable. He demanded an explanation from the United States for what he called “intentional killings.” Mr. Karzai said nine of the victims were children and three were women.

U.S. President Barack Obama responded, calling the shooting “tragic and shocking.” Mr. Obama expressed condolences to the families of the victims. He said he stands behind Defense Secretary Leon Panetta's call for a quick investigation and to hold anyone responsible accountable.

The U.S. defense chief assured Mr. Karzai that an investigation in under way and that a suspect is in custody. Panetta told the Afghan president in a phone call Sunday that those responsible will be brought to justice, echoing an earlier statement by the U.S. commander of NATO-led forces in Afghanistan General John Allen.

NATO officials say the suspect turned himself in following the early morning shooting rampage in a rural village in southern Afghanistan.

Afghan officials say the U.S. service member opened fire in at least three homes in Panjwai district early Sunday.

Many witnesses told VOA's Dari service that there were several attackers, but officials have mentioned only one suspect. A family member of one of the victims described the incident.

“When shooting was heard our dog started running. They killed him and came inside the house and put everyone in a room, shot them and then set everything on fire. Among those killed were four boys and four girls.”

NATO says it is treating those wounded in the attack at its medical facilities.

It remains unclear what might have motivated the service member's actions.

The shooting comes after weeks of tense relations between U.S. forces and their Afghan hosts, following the burning of Qurans at a U.S. military base.

Despite a U.S. apology for what it says was an accident, the burning of Qurans sparked violent protests and attacks across the country, killing dozens of people. There also has been a series of attacks by Afghan soldiers against foreign troops.

Afghanistan expert Andrew Wilder says these types of incidents together are very damaging for the U.S.-Afghanistan relationship, but he says they will not break the partnership.

“The need for a strong relationship there is so critically important both for Afghanistan and the U.S. We have had a lot of these kinds of incidents and they have all been weathered. And they are extremely unfortunate and make the job that much more difficult of sustaining a healthy relationship, but I don't think this will end up being seen as a tipping point.”

Wilder says recent tension is fueling sentiment in the United States that U.S. troops need to speed up their withdrawal. Under an international agreement, foreign combat forces are to leave Afghanistan by the end of 2014.