A U.S. service member shot dead at least 16 Afghan civilians in southern Kandahar province Sunday, renewing tensions between Washington and Kabul.

Afghan President Hamid Karzai called the shooting unforgivable. He demanded an explanation from the United States for what he called “intentional killings,” saying nine children and three women were among the victims.

U.S. President Barack Obama responded by calling the Afghan leader to express his shock and sadness at the incident. He extended condolences to the people of Afghanistan and said he stands behind Defense Secretary Leon Panetta's call for a quick investigation and to hold accountable anyone who was responsible.

In a phone call Sunday, Panetta assured Mr. Karzai that an investigation is under way and a suspect has been detained. He said 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 village near a military base. Many witnesses told VOA's Dari service there were several attackers, but officials have mentioned only one suspect.

Both NATO and Afghan officials have condemned the incident and expressed concern for the victims and their families.

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. The motivation behind the attack is unclear.

There is concern that Sunday shooting spree could further increase anti-American sentiment in Afghanistan. Relations between the two countries have been tense since U.S. soldiers inadvertently burned Qurans at Bagram military base near Kabul last month.

The incident sparked violent protests nationwide, killing dozens of people. There also has been a series of attacks by Afghan soldiers against foreign troops.

Afghanistan expert Andrew Wilder says attacks against Afghan civilians will fuel more anger against the U.S. but will probably not significantly alter the complicated 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.