An Afghan teenager who survived a rampage by a US soldier who killed 16 unarmed civilians last year testified about the pain of losing his grandmother, at the start of a sentencing trial for the man behind the carnage. The teenager, who was shot in the legs and whose sister was also seriously wounded, was among a group of Afghan victims of the violence flown to the United States to testify on the impact of the killings. "She loved me extra and every time I think of her I cry," the boy, whose name was given only as Rafiulla and who was described as aged about 15, said about his grandmother. The testimony at a US military base in Washington state came shortly after a jury of six military personnel was impanelled to decide the fate of US Army Staff Sergeant Robert Bales, who pleaded guilty to the killings in June. Bales, a veteran of four combat tours in Iraq and Afghanistan, has admitted to gunning down the villagers, mostly women and children, in night-time attacks on their family compounds in Kandahar province in March 2012. In exchange for his guilty plea, Bales will be spared the death penalty. The jury will determine whether he will spend the rest of his life in prison or be eligible for the possibility of parole after 20 years. Army prosecutors have said Bales acted alone and with chilling premeditation when, armed with a pistol, a rifle and a grenade launcher, he left his base twice during the night, returning in the middle of his rampage to tell a fellow soldier: "I just shot up some people." The proceedings at Lewis-McChord, a military base near Tacoma, Washington, are expected to last at least a week. In testimony, Mohammad Haji Naeem, 60, was asked by a military prosecutor what he was thinking when Bales shot him in the head. "What did I do, what have I done to you?" Naeem responded through an interpreter. The prosecutor, Army Lieutenant Colonel Jay Morse, asked Naeem how he felt when his son was shot and the Afghan became upset. "I'm leaving, for God's sake don't ask me any more questions," Naeem said, which drew his testimony to a close. The massacre marked the worst case of civilian killings blamed on a rogue US soldier since the Vietnam war and further eroded strained US-Afghan relations after more than a decade of conflict. Defence lawyers for Bales said on Monday that they would argue during the sentencing hearing that post-traumatic stress disorder and a brain injury were factors in the killings.