Government forces clashed with Al-Qaeda-affiliated militants backed by other armed Islamist groups in a village in Latakia province in the northwest.
The Syrian Observatory for Human Rights said the fighting broke out yesterday and raged through the night in Jabal al-Akrad district, a rare rebel stronghold in the province, which is the heartland of the ruling Assad clan.
Al-Qaeda's Syrian affiliate Al-Nusra Front and its allies were seeking to recapture the village of Dourine, which overlooks their positions in one of the district's main towns, Salma.
"The clashes yesterday led to 50 dead from both sides. There are many casualties because the fighters are facing off directly," said Observatory head Rami Abdel Rahman.
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"Al-Nusra managed to retake parts of the village, but the regime still controls the hilltop that overlooks Salma," he said.
The mountainous district of Jabal al-Akrad, which translates to Mountain of the Kurds, is the main rebel stronghold in Latakia province
Despite its name, it is populated mostly by Sunni Arabs who were early supporters of the protests against the Assad regime in 2011.
The region is one of multiple fronts in the four-year Syrian civil war, which has killed more than 210,000 people.
In the central province of Homs, at least five regime soldiers were killed today when a suicide bomber detonated an explosives-laden car at an army checkpoint, the Britain-based Observatory said.
Syria's state news agency SANA also reported the blast. It said four people were killed, without specifying whether they were soldiers or civilians.
The Islamic State jihadist group claimed a similar attack near a gas facility in the area in late December.