(AP) - An American soldier opened fire on villagers near his base in southern Afghanistan Sunday and killed 16 civilians, according to President Hamid Karzai who called it an "assassination" and furiously demanded an explanation from Washington. Nine children and three women were among the dead.

The killing spree deepened a crisis between U.S. forces and their Afghan hosts over Americans burning Muslim holy books on a base in Afghanistan. The burnings sparked weeks of violent protests and attacks that left some 30 people dead. Six U.S. service members have been killed by their Afghan colleagues since the Quran burnings came to light, but the violence had just started to calm down.

"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Karzai said in a statement. He said he has repeatedly demanded the U.S. stop killing Afghan civilians.

The violence over the Quran burnings spurred calls in the U.S. for a faster exit strategy from the 10-year-old Afghan war. President Barack Obama even said recently that "now is the time for us to transition." But he also said he had no plan to change the current timetable that has Afghans taking control of security countrywide by the end of 2014.

The tensions between the two countries had appeared to be easing as recently as Friday, when the U.S. and Afghan governments signed a memorandum of understanding about the transfer of Afghan detainees to Afghan control - a key step toward an eventual strategic partnership to govern U.S. forces in the country.