Security forces armed with heavy machine-guns shot dead 15 civilians in Rastan in the Homs region of central Syria on Thursday, a human rights activist said.

"Fifteen civilians were killed by heavy machine-gun fire in the town," the activist said, adding he had a list of the names. Security forces had sealed off the town, cutting off outside aid.

A witness, Talal al-Tillawi, meanwhile, said gunfire was also heard in Talbisa, another town in the Homs region, a flashpoint of an anti-regime revolt since mid-March.

"Security agents in army uniform are carrying out searches. They're smashing up everything they see, refrigerators, televisions, cars" in Talbisa, which like Rastan the army has encircled since Sunday, he said.