BEIRUT (Reuters) -- At least 20 people were killed on Thursday when Syrian security forces fired three mortar rounds at a Palestinian camp in Damascus, medical sources said.

Witnesses in the camp told Reuters by telephone that the mortars hit a busy street as people were preparing for the Ramadan meal to break their fasting.

Meanwhile in Hama, Syrian forces killed at least 50 people, among them 21 members of three families, during clashes with rebels in the central city of Hama, activists and residents said.

"During the clashes the army entered the neighborhood of Arbaeen and conducted raids, during which they killed members of three families," resident Abu Ammar told Reuters from the city.

He said among the dead were 10 members of the Kanan family, eight from a family named Okda and three from the Saffaf family.

Another resident said the clashes continued and rebels have appealed to fighters in other parts of the city for help.

Syrian forces besieged the Arbaeen district in the early hours of Thursday, residents said, adding that the army was using mortars and tanks to attack the neighborhood.

"We are unable to flee and many families are trapped inside their houses," a resident said.