WASHINGTON - The American soldier who allegedly killed 16 Afghan civilians on Sunday, including women and children, could face death penalty if convicted, US Defense Secretary Leon Panetta said on Monday.

"My understanding is that in these instances that could be a consideration," Panetta told reporters aboard his plane en route to Kyrgyzstan, when asked if death sentence could apply in this case.

He also gave a brief description of what happened, saying that the American soldier walked out of his base in Afghan southern province of Kandahar on Sunday morning, entered homes of nearby Afghan villagers and fired on civilians.

"Then at some point after that (he) came back to the forward operating base and basically turned himself in. Told individuals what happened," Panetta said.

He added his understanding is that the suspect had confessed to the killing, but the motive behind the rampage was still unknown.

The incident came when relations between Washington and Kabul have already been strained over the burning of copies of Quran by US troops last month.

Although Panetta characterized the shooting as an "isolated" event, it will almost further mar relations between the two countries when Washington is on its way to transfer security responsibility to Afghan forces and end combat mission by 2014.

The Obama administration has been trying to control the damage caused by the incident, stressing the "isolated" nature of the shooting spree and vowing to hold the suspect fully accountable.

US President Barack Obama said on Monday that the incident will not change the US strategy in Afghanistan and lead to immediate withdrawal of US troops from the country.