"If the government’s position is validated as a result of an inquiry, the international community can rest easy and the government will have been vindicated," he said.
"There is no justification for not moving ahead with such an investigation in view of the government’s confidence that such atrocities were never perpetrated by its armed forces," he added.
The images, which he described as "horrendous," indicate a serious violation of international law if found to be authentic, he said.
Alston also pointed out that he had asked permission to visit Sri Lanka on several occasions in recent years, but Colombo had not given him the green light.
The video footage, aired Tuesday by Channel 4 in Britain, was allegedly shot during the final stages of the army's defeat of Tamil Tiger separatists.