Ten people were killed in the Syrian central city of Homs and three others in the town of al-Quseir near the Lebanese border by security forces, opposition activists said Tuesday.
Heavy gunfire by security forces with the support of the army were heard in several neighbourhoods of Homs as security raids have been under way since the early hours of Tuesday, added the Local Coordination Committees of Syria (LCCS), DPA reported.
Bursts of machineguns and the hovering of helicopters were also heard amid complete blackout in most parts of the flashpoint city, according to LCCS.
Al-Arabiya broadcaster quoted unidentified eyewitnesses as saying that two people, including a 10-year-old boy, were injured in the clampdown in Homs.
More than 1,400 people have been killed by security forces since pro-democracy protests began in Syria in mid-March, according to local human rights advocates.
The government disputes the figure and blames "armed thugs" and foreign conspirators for the unrest.
The reports are hard to verify as the Syrian authorities have barred most foreign media and international human rights groups from entering the country.