Syrian activists say they have received reliable reports of heavy fighting between the security forces and defecting soldiers.
Sources inside Syria have told the London-based Syrian Observatory for Human Rights that dozens of army defectors are clashing with government forces in and around the southern town of Derra.
They initially said hundreds of new defectors were involved, but later revised down the number.
At least 18 people are reported to have died in the clashes.
The security forces are reported to be trying to stop Syrians from joining the general strike called by the opposition as a way of further weakening the Government's finances, according to the BBC.
Activists say the strike has been widely observed in Derra and Homs.
They hope it will build across the country in the coming days.