BEIRUT: Syrian regime forces pounded the rebel-held town of Rastan in central Homs province on Thursday, killing three civilians, as four people were "summarily executed" in a province bordering Turkey, a monitoring group said.

The Syrian Observatory for Human Rights said a total of at least 13 people were killed across the country, including the four executed in Idlib province of northwest Syria.

Government forces have been trying to overrun Rastan for 11 straight days, after rebel fighters from the battered city of Homs regrouped in the town which straddles the main highway linking Damascus to the north.

Speaking to AFP via Skype on Wednesday, Abu Rawan, an opposition activist in Rastan, said there was no electricity in the besieged town, and a shortage of food and water. He was not reachable for comment on Thursday morning.

Elsewhere in the violence-swept country, four civilians were summarily executed in a field near Basamis town in the province of Idlib.

"Eight civilians were killed in a field near Basamis this morning, including four who were detained from their homes and summarily executed," according to the Britain-based Observatory.

"The Observatory strongly condemns the summary execution of the four people, in contravention of international treaties signed by the Syrian authorities," it said in a statement.

"The Syrian security and military authorities do not have the right to summarily execute people, even if they are fighters," it said.

Four other people were killed in clashes in Idlib between regime troops and rebel forces, the Observatory added.

In eastern Deir Ezzor province, one civilian and a regular army soldier were killed, the watchdog said.

"Regime forces killed a young man at dawn in Quriya town in Deir Ezzor," it said, adding that a soldier died in clashes between rebel forces and regime troops in the same town.

Also on Thursday, the corpses of six civilians kidnapped earlier this week were discovered in the central province of Hama, the Observatory said.

"The bodies of six people from one family, including children, were found in the province of Hama," the watchdog said, adding that the corpses were of a man, a woman and their four children.

More than 12,600 people have been killed in Syria since a revolt against President Bashar al-Assad's rule broke out in March 2011, including nearly 1,500 since a UN-backed truce took effect on April 12, says the Observatory.