Up to sixty Shia Muslims have been killed in an attack by opposition fighters in the eastern Syrian town of Hatla. A video posted online on Tuesday, entitled “The storming and cleansing of Hatla”, showed fighters waving the black Salafi flag and celebrating.

“We have raised the banner ‘There is no God but God’ above the houses of the apostate rejectionists, the Shia,” the voice of the cameraman says.

The British-based Syrian Observatory for Human Rights claimed that thousands of rebels participated in the killings, although the footage of the attacks pictured just a handful of fighters. The death toll could not be independently verified.

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Activists denied that the killings were sectarian in nature, claiming instead that they were a response to an attack on a Free Syrian Army checkpoint by 30 armed villagers from Hatla. The activists claim the opposition fighters were incited by the Assad regime’s decision to arm the population of Hatla, one of the only Shia villages in an area that has been firmly under rebel control for more than a year.

However, the language used in the videos is explicitly sectarian. “This is the Shia, this is the Shia carcass, this is their end,” the cameraman says as a blanket is lifted from one of the victims. That video, posted by a pro-Assad group, indicates those responsible were non-Syrians, possibly from Kuwait.

A Syrian government official spoke of a “massacre against villagers in which older people and children were killed”.

Lebanese militant group Hezbollah, which has recently joined the fighting in support of the Assad regime, called the incident an act of sectarian cleansing that was inspired by Sunni clerics in the region.

In a speech in front of the Lebanese embassy in Kuwait, Salafi Sheikh Shafi al Ajmi, one of the most outspoken supporters of the Syrian opposition, said the attack was retaliation for the recent fighting in Qusayr. The town on the Lebanese border was recaptured from rebel control by the Syrian army and Hezbollah fighters last week. He said Syria would be “a burying ground” for Hezbollah.

Meanwhile, a Syrian government helicopter fired three missiles into the Lebanese border village of Arsal, injuring one. The rockets hit the centre of the predominantly Sunni town, which supports the uprising against Syrian president Bashar al-Assad, himself a Shia.

Arsal lies just 15km across the border from Syria and is currently home to more than 20,000 refugees, many of them from Qusayr. Although the helicopter strike is the first attack on an urban centre inside Lebanon by Syrian forces, the Lebanese border area has been facing assault from opposition fighters for months.

Six rockets also hit the town of Hermel, a Hezbollah stronghold, on Tuesday, wounding four people and killing one. Blaming the attacks on the rebel presence in Arsal, militants from Hermel killed Ali Ahmed Hujeiri, an Arsal resident, in what appears to have been an ambush.

Reporters at the scene said relatives claimed the man had been shot in the stomach, but the perpetrators had scattered shrapnel on the road to make it look like a shell strike. As the body had already been buried, the reports could not be corroborated.

The Hujeiris are the most influential family in Arsal; the victim’s brother is the mayor while another brother, prominent Salafi sheikh Mustafa Hujeiri, called for his supporters to engage in jihad and avenge his brother’s death. The attacks are the latest in a conflict that has killed more than 80,000 people, according to the United Nations.