Syrian troops and armed groups described by one resident as "death squads" have laid siege to the city of Homs since Monday night, sources said.
Syrian rights activist Rami Abdulrahman, director of the London-based Syrian Observatory for Human Rights, said that the city had been the scene of intense battles since security forces stepped up a crackdown on Monday night.
Homs has been a focal point of the uprising against President Bashar al-Assad since the military stormed its districts two months ago to crush anti-government protests.
"Thirteen civilians were killed yesterday and today in several parts of Homs when the army opened fire as it carried out an operation in the city," he said.
Abdulrahman said the fighting in Homs erupted after three regime supporters kidnapped last week were killed and their dismembered bodies were returned to their relatives.
"These clashes are a dangerous development that undermines the revolution and serves the interests of its enemies who want it to turn into a civil war," he said.
Al Jazeera cannot independently verify events in Syria because of restrictions on reporting.
Updating the Reuters news agency by phone on Tuesday, a Homs resident said, "There are troops and armored vehicles in every neighborhood. The irregular forces with them are death squads.
"They have been firing indiscriminately since dawn with rifles and machine guns. No one can leave their homes."
Activists say the Syrian government's crackdown on pro-democracy and anti-regime protests has left more than 1,400 civilians dead and thousands more imprisoned.
PHOTO CAPTION
Protesters shout slogans during a demonstration against Syria's President Bashar al-Assad in the eastern Syrian town of Deir al-Zour, near the border with Iraq, July 16, 2011.
Al-Jazeera