More than 16,500 people have been killed in violence since an uprising against President Bashar al-Assad broke out in March last year, the Syrian Observatory for Human Rights said on Monday.

The 16,507 dead comprise 11,486 civilians, 4,151 government troops and 870 army defectors, the Britain-based watchdog told AFP.

In its running tolls, the Observatory counts as civilians those rebel fighters who are not defectors from the army.

The United Nations no longer publishes its own estimates of the death toll.

In violence on Monday, at least five civilians were killed, the Observatory said.

Four died in the eastern province of Deir Ezzor, when the car they were travelling in was shelled. A fifth was killed when troops shelled the rebel-controlled central town of Rastan.

An activist in Syria's third largest city of Homs told AFP via Skype that many civilians remained trapped in shelling of the Jourat al-Shiah, Khalidiyeh and Old City neighbourhoods.

"Many neighbourhoods of Homs are still under siege, and it is really hard for us to get food or medicines in," said Khaled al-Tellawy.

"Field doctors are amputating the limbs of the injured because they have no equipment to treat them with, and they can't be smuggled out."

The besieged neighbourhoods of Homs are among a string of areas that have fallen into the effective control of rebel fighters, the Observatory said.

Late last month, the International Committee of the Red Cross and the Syrian Arab Red Crescent made an unsuccessful attempt to reach the trapped civilians in Homs.