Syrian rebels, including Sunni extremists, stormed a village and battled pro-regime militiamen, killing more than 60 Shiite fighters and civilians in an attack steeped in the sectarian hatreds that increasingly characterize the civil war, activists said Wednesday.

The victorious fighters raised black Sunni Islamist flags over the eastern village of Hatla. In amateur videos, the fighters vented anti-Shiite slurs and fired in the air.

"The homes of the infidel Shiites were burned," the voice behind the camera in one video shouted as smoke rose in the background from several houses.

The attack Tuesday on Hatla, in Syria's Deir el-Zour region near Iraq, underlined the increasingly sectarian nature of the conflict. The regime called it a "massacre."

The U.S. and other Western nations have been hesitant to arm the outgunned rebels because of Sunni jihadi radicals among their ranks.

"The motivations and circumstances surrounding this massacre remain unclear, but the United States strongly condemns any and all attacks against civilians," said State Department spokesman Jen Psaki.

Most of the armed rebels in Syria are from the country's Sunni majority, while Assad has retained core support among his own Alawite sect, an offshoot of Shiite Islam, along with Christians and Shiites.

Sectarian bitterness has grown. Each sect has been accused of massacres against the other.