Syrian forces are reported to have shot dead more than 70 army deserters as they tried to join the revolt against President Bashar al Assad.
The development came as Syria signed a protocol accepting the deployment of Arab observers on the ground.
The BBC reports it is impossible to independently verify the accounts, as foreign media are banned from reporting in Syria.
However, if they are true, it would have been one of the bloodiest days since the uprising began in March this year.
The Syrian Observatory for Human Rights, based in London, and activist groups in Syria say a large number of soldiers who are planning to desert Idlib province in the north-west were cut down by machine gun fire as they tried to flee.
Activists say the death count for the day was swollen by further killings of civilian protesters in other areas, including Homs to the north of Damascus and Deraa in the south.