Syrian opposition forces attacked an east Syrian village, killing an estimated 60 Shiites, the Syrian Observatory for Human Rights said on Wednesday.
The majority of those killed in the attack were pro-government fighters, the Observatory added.
The Syrian government spoke out on the attack on Wednesday morning, calling It a “massacre”.
The event took place late on Tuesday evening in the Deir El Zour province, the Observatory said, noting that the attacks outline the truly sectarian nature of the now two year old Syrian civil war.
The Observatory, a British based organisation that relies on an intricate web of professionals working and living within Syria for its information, said that at least 60 people had been killed in the small village of Hatla, close to the Iraqi border.
According to insider reports, thousands of rebels took part in the offensive  on Hatla and at least 10 died during the fighting.
The state-run media, loyal to Syrian President Bashar Assad, reported that the rebels "carried out a massacre against villagers in which older people and children were killed,” according to AP.
Considered a successful offensive by the rebels, their move comes a week after the Syrian regime dealt them a significant blow when it seized the strategic town of Qusayr .
Near the Lebanese border, the battle for Qusayr saw the increased involvement in Lebanese Shiite Hezbollah troops becoming embroiled in Assad's war.
Hezbollah's involvement, and it's ultimate choice to choose religion over state, has caused many to believe that an all-out civil war will engulf Lebanon, as the country becomes increasingly more embroiled in Syria's brutal sectarian politics .
Assad belongs to the Alawite sect, an offshoot of Shiite Islam. Pairing up with the big Shiia players in the region, Hezbollah and Iran, the pro-regime forces have been batting the Sunnis for control of the country. The majority of Syria's population is Sunni.
An activist based in Deir El Zour told AP over Skype that the rebels attack on Hatla was in retaliation to the killing of four rebels on Monday.
He added that as a result of the rebel attack, an estimated 150 shiites from Hatla had fled the area, seeking refuge in the government-held village of Jafra across the Euphrates River, according to AP.
"The situation in the village is quiet and the Free Syrian Army is in full control," he told AP, noting that the village of Hatla has been under rebel control for over a year. He said that some residents had been moving towards a revolt against the rebel control and had been accumuulating arms, which stimulated Tuesday's attack.
Also Wednesday, the Observatory reported that heavy clashes between rebel and regime forces were ongoing in the Wadi Sayeh neighourhood of the central city of Homs.
The battles, according to the Observatory, were a regime attempt to separate and isolate two of the rebel-held suburbs of the city.
In the aftermath of its triumph in Qusayr, Syrian regime forces have focused their attention on the city of Homs, which has been a rebel-stronghold for much of the conflict.
Homs is particularly significant due to its geographical location – due to its centrality, it links Damascus with regime held areas in the rest of the country.