A graphic video allegedly showing a Syrian rebel cutting open a pro-government fighter’s chest and biting into one of his internal organs sparked outrage Tuesday, reviving concerns about the makeup of opposition forces even as the United States warned that it is ready to bolster its support for them.

During a visit to Stockholm, Secretary of State John F. Kerry said it would be a “gross miscalculation” by Syrian President Bashar al-Assad to back out of peace talks brokered by Washington and Moscow, saying without elaboration that such a move would prompt additional aid for rebel forces.

Kerry’s warning came despite the concerns raised by the video, which stands out even among the daily streams of gruesome images that emerge from the conflict. In it, a man identified as a rebel commander bends over a body and cuts open the chest with a knife before standing to face the camera with an organ in each hand. “I swear to God, we will eat your hearts and your livers, you soldiers of Bashar the dog,” he declares, before lifting the flesh to his mouth and biting it, to shouts of “God is great!”

The chilling scene points to the increasingly sectarian nature of the Syrian conflict , which has been marked by revenge killings along religious lines, and highlights the challenges complicating efforts to assist fractured opposition military groups. British Prime Minister David Cameron said during a visit to Washington on Monday that his government would double its military support to Syrian rebels, but the Obama administration has remained cautious.

The rebel in the video was widely identified as Khaled al-Hamad, nicknamed Abu Sakkar, a commander of the Omar al-Farouq Brigade; that force is a more militant offshoot of the Farouq Brigades, a prominent armed faction formed in the central city of Homs. In an interview with Time magazine, Abu Sakkar said he had no regrets, explaining that his men had discovered a video clip on the soldier’s cellphone of a woman and her two daughters naked. “He was humiliating them, and sticking a stick here and there,” he said.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

The New York-based Human Rights Watch said a longer version of the video that has not been publicly circulated includes a slur against Alawites, the minority Shiite sect to which Assad belongs.

“Oh my heroes of Baba Amr, you slaughter the Alawites and take their hearts out to eat them,” the commander reportedly adds, referring to a former rebel stronghold on the outskirts of Homs.

The Britain-based Syrian Observatory for Human Rights said Tuesday that as many as 41,000 Alawites have been killed in the two-year-old conflict. The group revised its total death toll to at least 94,000 but said the number may be as high as 120,000. The United Nations said in February that about 70,000 had been killed.

Human Rights Watch called for the U.N. Security Council to support the referral of war crimes allegations against both sides to the International Criminal Court. The Syrian Opposition Coalition said it strongly condemned the “horrific and inhumane” act, adding that the perpetrator would be brought to justice.

The video follows similarly graphic battlefield material released in recent months, including an image that purports to show a rebel barbecuing the severed head of a government fighter.

“It is indicative of how absolutely fierce this conflict is becoming,” said Andrew Tabler, a senior fellow at the Washington Institute for Near East Policy. “It challenges the notion that a political settlement is viable at this time.”

The Obama administration has said that it will not rule out arming the rebels, but officials say they will not consider doing so until after peace talks are held. Syria’s information minister, Omran al-Zoubi, said Tuesday that his government would not agree to talks until more details were clear, adding that Assad’s future role should be decided only at the ballot box.

“If President Assad decides to miscalculate again about that, as he has miscalculated about his own country’s future over the course of the last years, it is clear the opposition will be receiving additional support, there will be additional efforts made, and, unfortunately, the violence will not end,” Kerry said.