A U.S. Army sergeant opened fire on villagers in southern Afghanistan early Sunday, killing 16 people — mostly women and children.

The slayings in two villages in Kandahar province, described as the worst atrocity committed by U.S. forces during the 10-year Afghan war, came amid already deepening tensions between Washington and Kabul.

Afghan President Hamid Karzai called the shooting “an intentional killing of innocent civilians that cannot be forgiven.” He demanded an explanation from the United States, saying that 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, identified as an 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 told VOA's Dari service there were several attackers, but officials have mentioned only one suspect.

Acting U.S. Ambassador to Afghanistan James Cunningham expressed his deepest condolences to the families of the victims. He said U.S. forces are providing the highest level of care for those injured, and that the incident is under investigation.

Both NATO and Afghan officials have condemned the incident and expressed concern for the victims and their families. The motivation behind the attack is unclear.

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.”

There is concern that Sunday's 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 probably will not significantly alter the complicated partnership.

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.