A school employee was killed and several students hurt, four seriously, when a shell exploded near a school bus, activists said.
Security forces in several other parts of the country fired on crowds holding overnight demonstrations, causing casualties, they said.
The new attack using military forces pointed to Mr Assad's determination to crush the two-month-old revolt, despite US and European sanctions, including an EU assets freeze and a visa ban on Mr Assad and nine members of his regime.
The uprising, which began in mid-March, is posing the most serious challenge to his family's 40-year rule. What began as a disparate movement demanding reforms has grown into an uprising seeking the overthrow of Mr Assad. Human rights groups say more than 1,000 people have been killed in the crackdown.
Yesterday's attacks targeted the towns of Rastan, Talbiseh and Teir Maaleh in the central province of Homs. Authorities had sealed off and isolated the towns by closing roads and cutting phone service, activists said.
"The towns are under siege," one said. Residents have held anti-regime protests since the start of the uprising. These have increased recently, with crowds on the streets day and night.
The Syrian Observatory for Human Rights said two people were killed in Rastan and four were wounded in Talbiseh. There were no immediate reports of casualties in Teir Maaleh.
An activist said at least three people were killed in Tabliseh.