If the residents’ claims are verified, they would reflect the first large-scale army defections and fighting among security forces. That would pose a grave challenge to the government of President Bashar al-Assad, who has relied on security forces to crush an unprecedented popular revolt against more than four decades of iron-fisted rule by his family.

But even the government’s version of events suggests that Mr. Assad’s troubles are escalating, since the protests against the government had, until then, been mostly peaceful.

It has been difficult to verify either account, and to sort fact from spin, since Syria has kept most foreign journalists out of the country.

Secretary of State Hillary Rodham Clinton, in Zambia, said Syria’s government was “engaged in horrific, revolting attacks against its own people” and found herself forced to defend the lack of forceful international action. She said regional leaders did not support the sort of intervention under way in Libya. “The region, however, is trying behind the scenes to get the government to stop,” she said in an interview with “Africa 360,” a news program.

Image Many residents from Jisr al-Shoughour and nearby villages have fled into the mountains and toward the Turkish border. Credit... The New York Times

Residents of Jisr al-Shoughour who were reached by phone on Saturday said that the eastern part of the town was too dangerous to enter because of heavy fire from nearby military units. The northern section of the town was safer, allowing more people to flee, but residents said they had to watch out for helicopters opening fire on “anything that moves.”