Syrian security forces and affiliated gunmen killed more than 340 civilians, the vast majority of them from the Alawite minority, over the last two days, Rami Abdulrahman of the Syrian Observatory for Human Rights told Reuters on Saturday.
Other Syrian sources have reported casualties somewhere between 180-200.
Syrian state media on Thursday cited an interior ministry source as saying "individual violations" had occurred during a government operation to crack down on terrorists linked to the ousted Assad regime and said it was working to address the incidents.
Reuters could not independently verify the reports.
Syrian security forces battled for a second day on Friday to crush a nascent insurgency by fighters from Bashar al-Assad's Alawite sect, with scores reported killed as the Islamist-led government faced the biggest challenge yet to its authority.
They included at least two dozen male residents of the Alawite town of Al Mukhtareyah killed by gunmen on Friday, the Observatory and two Alawite activists said, citing contacts in the region and video footage from the scene.
In his first comments on the violence, interim President Ahmed al-Sharaa said government forces would pursue "remnants" of the ousted Bashar al Assad government and bring them to trial. He also said that those who assaulted civilians would be held accountable.
"We will continue to pursue the remnants of the fallen regime .. . We will bring them to a fair court, and we will continue to restrict weapons to the state, and no loose weapons will remain in Syria," Sharaa added in a pre-recorded speech.
Syrian authorities said the violence began when remnants loyal to Assad launched a deadly and well-planned attack on their forces on Thursday.
The violence has shaken Sharaa's efforts to consolidate control as his administration struggles to get US sanctions lifted and grapples with wider security challenges, notably in the southwest, where Israel has said it will prevent Damascus from deploying forces.
The oil-rich northeast of the country also remains outside state control, held by a US-backed Kurdish-led group.
Syrians took to the streets on Friday to rally in support of the government in Damascus and other cities.
Iran, formerly Assad's closest ally, said it "strongly opposes insecurity, violence, killing and harming innocent Syrians from every group and tribe."
Images from Al Mukhtareyah showed at least 20 men lying in close proximity - some bloodied - by the side of a road in the town center. Reuters was able to verify the location of the video, but it did not verify when or by whom it was filmed.
Alawite activists, speaking on condition of anonymity, blamed the killings on gunmen affiliated with the Islamist ruling authorities.
Syrian state news agency SANA, citing a security source, said "individual violations" had been perpetrated after unorganized crowds had headed to the coastal region following the attacks on government security personnel.
"We are working to stop these violations," the source said.
Curfew
The violence spiraled on Thursday when the authorities said groups of Assad-aligned militias targeted security patrols and checkpoints in the Jableh area and the surrounding countryside before spreading.
Moussa al-Omar, a Syrian media figure close to the country's new leadership, told Reuters that tens of thousands of fighters in Syria's newly constituted security forces had been deployed to the coast in the operation, and order had been largely restored as of Friday night.
He said the crackdown was "a message to anyone in the south or east of Syria that the state ... is capable of a military resolution at any time even as it seeks peaceful solutions."
Curfews were declared on Friday in the coastal cities of Tartous and Latakia, SANA said. Security forces mounted combing operations in both cities and nearby mountains.
Alawite activists say their community has been subjected to violence and attacks, particularly in rural Homs and Latakia, since Assad was overthrown in December after decades of repressive family rule and civil war.
While Sharaa has pledged to run Syria in an inclusive way, no meetings have been declared between him and senior Alawite figures, in contrast to members of other minority groups.