Britain and the US have warned they will rethink support for Syria's main opposition group if it fails to join peace talks, a Syrian source has said.

Syrian forces loyal to President Bashar al-Assad killed dozens of rebel fighters who tried to break an army siege of the central city of Homs, state media and a monitoring group said.

The first batch of Syria's chemical weapons stockpile was shipped out of the country on Tuesday from the port of Latakia. The move is the culmination of an international effort to dismantle the Assad regime's chemical weapons program which will have to be followed through with continued chemical exports for any chance of success.