In Homs, the country's third-largest city, a siege by President Bashar Assad's security forces continued in retaliation for the killing of several members of his Alawite Muslim sect last week, the Los Angeles Times reported.
Footage posted to the Internet showed tanks and artillery shelling Sunni mosques and neighborhoods.
The opposition says the regime is trying to turn the uprising into a sectarian conflict.
"That is what the regime wants everyone to believe so they become afraid," said a Western diplomat in Damascus, speaking on condition of anonymity. "And that is the challenge for the opposition. They have to opt out of fearful propaganda and show that they are unified. But I think we've seen very little sectarian violence among ordinary citizens."
A witness near Homs told the Times the army was fortifying the Lebanese border to keep the wounded from being evacuated to Lebanon.