The Syrian army stepped up its military operation in the central city of Homs which has become a focal point for pro-democracy protests, activists and residents said on Thursday.
Shooting and explosions were heard in the old district of Bab Sbaa, residents said.
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"There are casualties and many people have been arrested. We are very scared," a resident in Homs, who asked to be referred to only by his first name, Ahmed.
A resident in another neighborhood said local hospitals were calling for blood donations after they received wounded people from Bab Sbaa.
"There is heavy military deployment in Homs; military checkpoints are everywhere in the city. There is heavy shooting in Bab Sbaa, one house was burned and the humanitarian conditions are dire," the Syrian Observatory for Human Rights said in a statement.
An activist in Homs said the army had raided houses in Bab Sbaa. A resident in the city told Reuters by phone that the army fired at worshipers in eastern Khalidiya district as they left Khaled Ibn al-Walid mosque in the early hours of the morning.
It is hard to verify witness accounts as Syrian authorities have expelled most international media organizations.
Homs has been a center of the protests against Syrian President Bashar Assad's 11-year rule and in support of political freedoms.
Tensions have run high between the majority Sunni inhabitants and members of the Alawite minority, the same sect as Assad, creating fears that a peaceful pro-democracy movement could take a dangerous turn towards a sectarian conflict.
The return of the mutilated bodies of three Alawite men to their relatives had led to fighting between armed residents in Homs this week, the Observatory said on Monday.
The total death toll in Homs since the weekend has risen to at least 33, activists and residents said. It is difficult to verify whether people were killed by government forces or feuding residents.
Human rights organizations say at least 1,400 civilians have been killed since the protests against Assad erupted in March.