Videos posted on YouTube and obtained by satellite television showed bodies in the streets of the city of Homs and in a southern village, apparently after troops had opened fire on protesters the day before.
But the worst of the violence has occurred in the southern town of Daraa, where the revolt began in March and which has been under siege by the Syrian army since Monday.
Reports from the town Saturday said that the Syrian military had overrun the Omari mosque in the heart of the old city where regime opponents had taken refuge, turning it into a makeshift hospital for those injured in the assault.
A resident contacted by satellite telephone said the onslaught began shortly before dawn, when soldiers backed by tanks and helicopters surged through the narrow streets of the old city and stormed the mosque. He said he could see snipers positioned in the mosque's minaret from the window of his home nearby, and that he thought at least six people had been killed.
The use of helicopters, which could not be independently confirmed, would mark a first in the effort to suppress the Syrian uprising. The Associated Press quoted a different witness, Abdullah Abazeid, as saying three helicopters were used in the assault.
The resident who spoke by satellite telephone asked that his name not be used because he fears the consequences. He described dire conditions inside the town, which is running out of food and medicine and has had its water, electricity and telephones cut off since Monday.
With snipers positioned on buildings and at intersections throughout Daraa, residents do not dare leave their homes and communicate with one another by calling through their windows. That way, news of events in one part of town is transmitted, street by street, to people living in another, and it is common to hear the calls of neighbors echoing from house to house through the deserted streets, he said.
Yet even as the crackdown intensifies, people in Daraa remain determined to sustain their opposition movement. “We will stand firm until the last child,” he said.
An exact count of the people killed since tanks rolled into the town on Monday is impossible, but earlier in the week residents described bodies left in the streets because people were too afraid to retrieve them. On Friday, human rights groups said 83 bodies had been stashed in a makeshift morgue because families were unable to go to cemeteries to bury them.
An amateur video obtained by Al Jazeera on Saturday appeared to confirm the reports. It showed dozens of bodies inside a container, although the video's contents could not be independently confirmed. Verifying reports that come out of Syria is almost impossible because the government has refused to admit all but a handful of foreign journalists, and the movements of those who are there are restricted.
The latest violence brought to 535 the number of deaths recorded since the unrest began in mid-March, according to Rami Abdulrahman of the Syrian Observatory for Human Rights. Among them are 36 people who were killed on the outskirts of Daraa when troops fired into protesting crowds from nearby villages who were attempting to enter the town, he said.
Although the scale of the crackdown in Syria is approaching Libyan proportions, the international community has been muted in its response, reflecting widespread concerns that the regime's collapse could destabilize the region.
The Obama administration announced sanctions against three Syrian officials on Friday, freezing the U.S. assets of the president's brother, Maher al-Assad, who commands the forces leading the assault against Daraa, intelligence service director Ali Mamluk and Atif Najib, a cousin of the president and a leading official in Daraa.
But it is thought unlikely that the three have any U.S. assets because existing sanctions already prohibit financial transactions between the United States and Syria, and activists said the measure would have little impact.
Special correspondent Leena Saidi contributed to this report.