BEIRUT (AP) — A U.S.-supported Syrian rebel group said Wednesday it will open an investigation into the beheading of a Syrian boy on spy accusations, calling it an "individual mistake" that does not represent the overall policies of the group.

The Nour el-Din al-Zinki group, a relatively moderate group that fights the Syrian government and the Islamic State group, condemned the beheading which was captured on video and triggered an instant backlash against the group.

It is not clear who the boy was or why he was beheaded on camera. He was identified by activists on social media as Mahmoud Issa, a 12-year-old Palestinian. The Syrian government also denounced the killing in a statement to the United Nations, saying the boy was a Palestinian from a refugee camp near Aleppo.

The incident allegedly occurred in Handarat, near Aleppo, on Tuesday. A young boy is seen on the back of a pickup truck surrounded by armed men who accuse him of being a spy and a member of the Quds Brigade, a pro-government Palestinian militia. He is then beheaded with a knife by one of them.

Nour el-Din al-Zinki, in its statement, said it will investigate the incident and expedite punishment. It said everyone involved in the violation was arrested and are being investigated. The group, which operates mainly in Aleppo, has received support in the past from the United States.

Amnesty International said the video is the latest "abhorrent signal" that opposition groups are carrying out serious abuses with impunity.

"This horrific video showing the beheading of a boy suggests some members of armed groups have truly plumbed the depths of depravity. It is yet another gruesome example of the summary killing of captives, which amounts to a war crime," said Philip Luther, Director of Amnesty International's Middle East and North Africa Program.

U.S. Secretary of State John Kerry said he has seen the "appalling report" but could not confirm it happened. "Obviously we're very concerned certainly if it's accurate," he said, adding that if the report is proven it would "give us pause about any assistance or, frankly, any further involvement with this group."

Lebanon's Hezbollah group, whose fighters are in Syria fighting alongside President Bashar Assad's forces against the rebels, also pounced on the beheading. In a statement issued in Beirut, it described the beheading as a "disgusting crime" by "so-called moderate rebels backed by the U.S. and Saudi Arabia."

Meanwhile, an explosion struck a building in a southern town in Syria Wednesday, but there was no immediate word on casualties and there were conflicting reports on the cause of the blast.

The Britain-based Syrian Observatory for Human Rights said an Israeli warplane targeted the building in the town of Baath, on the edge of the Israeli-occupied Golan Heights, without saying how it knew it was an Israeli warplane. Israeli warplanes and artillery have struck southern Syria in the past, and fighters from the Iranian-backed Lebanese Hezbollah militia are believed to operate in the area.

But the Military Media of Hezbollah denied an Israeli airstrike and claimed that the blast was caused by two rockets fired by members of al-Qaida's affiliate in Syria, the Nusra Front.

Later, Hezbollah's Al-Manar TV said Syrian government troops retaliated, hitting a Nusra Front vehicle from which the rockets were fired.