Syrian rebels have warned two Christian towns in the central region of Hama they will be attacked if they do not evict pro-regime fighters, according to a video distributed by a watchdog on Saturday.

The footage released by the Syrian Observatory for Human Rights and entitled "Warning mainly to Christian cities in the province of Hama", shows seven Islamist fighters armed with Kalashnikovs.

"We issue this warning so that you expel gangs of (President Bashar al-) Assad and shabiha (pro-regime militia) from your towns and convince them not to bomb our villages and families," said a fighter who identified himself as Rashid Abul Fida, head of the Al-Ansar Brigade in Hama.

"If not, we will immediately attack the hideouts of Assad's gangs and shabiha."

Observatory director Rami Abdel Rahman said the towns of Mharda and Al-Sqilbiya to which the rebel video was addressed both had pre-war populations in the tens of thousands but most of their residents had already fled, taking refuge in the coastal province of Tartus.

Rebels on Monday launched an all-out assault on army positions across Hama, according to the Observatory.

"Since the start of operations to liberate Hama province, the (rebel) Free Syrian Army has attacked the tyrant's soldiers and shabiha in the majority of towns and villages," said Abul Fida.

There are some 1.8 million Christians in Syria, many of whom have remained neutral in the country's conflict. Others have taken Assad's side, for fear of the rise of Islamism.