Residents said on Thursday about 40 tanks and troop carriers had deployed about 7 km (4 miles) from Jisr al-Shughour. Activists and residents say the violence began after a mutiny among security forces who refused to fire at protesters.
Turkey's Red Crescent said it was setting up a second camp near the border to shelter people still crossing from Syria to escape the military build-up.
Turkish Foreign Minister Ahmet Davutoglu said on Thursday 2,400 people had already entered Turkey
"Jisr al-Shughour is practically empty. People were not going to sit and be slaughtered like lambs," said one refugee who crossed on Wednesday and who gave his name as Mohammad.
Syria has barred most independent media from the country, making it difficult to verify accounts of the violence.
Assad, 45, has promised reforms even while cracking down on unrest buffeting the country that has become the gravest threat to his 11-year authoritarian rule. Friday prayers have been a focus of protests throughout the revolt.
The governing board of the U.N. nuclear watchdog reported Syria to the Security Council on Thursday for covert atomic work, a U.S.-led move coinciding with Western condemnations of Damascus' crackdown on opposition protests.
Russia and China voted against the proposal at the International Atomic Energy Agency, underscoring big-power divisions that may rule out any follow-up punitive measures for the time being.