BALANDI, Afghanistan (AP) — Moving from house to house, a U.S. Army sergeant opened fire Sunday on Afghan villagers as they slept, killing 16 people — mostly women and children — in an attack that reignited fury at the U.S. presence following a wave of deadly protests over Americans burning Qurans.

The attack threatened the deepest breach yet in U.S.-Afghan relations, raising questions both in Washington and Kabul about why American troops are still fighting in Afghanistan after 10 years of conflict and the killing of Osama bin Laden.

The slayings, the worst atrocity committed by U.S. forces during the Afghan war, came amid deepening public outrage spurred by last month’s Quran burnings and an earlier video purportedly showing American Marines urinating on dead Taliban militants.

The Quran burnings sparked weeks of violent protests and attacks that left some 30 Afghans dead, despite an apology from President Barack Obama. Six U.S. service members were also killed by their fellow Afghan soldiers, although the tensions had just started to calm down.

According to U.S. and Afghan officials, Sunday’s attack began around 3 a.m. in two villages in Panjwai district, a rural region outside Kandahar that is the cradle of the Taliban and where coalition forces have fought for control for years. The villages are about 500 yards (meters) from a U.S. base in a region that was the focus of Obama’s military surge strategy in the south starting in 2009.

Villagers described cowering in fear as gunshots rang out as a soldier roamed from house to house firing on those inside.