Several civilians also reportedly died.
Activists say government forces began bombarding the town on Sunday night.
According to the UK-based Syrian Observatory for Human Rights, rebels fought back, inflicting damage and casualties on the Syrian forces. The claims cannot be verified.
The latest violence strikes a blow to the UN-backed ceasefire, as new observers joined the existing mission in Syria. More than 100 inspectors have now been deployed.
Rastan, reported to be a major source of army conscripts, has changed hands several times as both sides in the conflict battle for control.
The EU has imposed another round of sanctions on the Syrian government.