Moscow was deeply shocked at the murders and denounced them as "inhumane acts" and "one of the gravest war crimes," said Konstantin Dolgov, the foreign ministry's commissioner for human rights, democracy and the rule of law.

"We expect those responsible will be severely punished and the command of the international forces will adopt efficient measures not to let similar accidents happen again in the future," Dolgov said.

At least 16 civilians, including women and children, were reportedly killed by the U.S. soldier, who walked out of his military base Sunday morning and opened fire at villagers in the southern Afghan province of Kandahar.