The head of Syria's military police defects from the army - which he says has become a "gang of killing" - and joins the country's rebels, amid reports of a fatal government shelling attack.

Please wait while this video loads. If it doesn't load after a few seconds you may need to have Adobe Flash installed.

Syria military police chief defects to join rebels (R)

Syria's chief of military police pledged allegiance to the uprising against President Bashar al-Assad in a video posted in YouTube, which was confirmed by a Syrian security source.

The former major general said he was defecting because of the number of civilian casualties.

"I am General Abdelaziz Jassim al-Shalal, head of the military police," said the police chief in a recorded video message. "I have defected because of the deviation of the army from its primary duty of protecting the country and its transformation into gangs of killing and destruction."

The high-level defection, while not a strategically significant development in the 21-month-old conflict, will be a blow to morale for President Assad's forces, which are hitting back at a string of rebel advances across the country.

Read more on Syria from Chief Correspondent Alex Thomson

Army 'committed massacres'

It comes amid reports of fatal shelling by the government in the northern province of Raqqa, the UK-based Syrian Observatory for Human Rights said killed 20 people.

A Syrian security source confirmed the defection but played down its significance. "Shalal did defect but he was due to retire in a month and he only defected to play hero," the source said.

Wearing a camouflage uniform with red officer insignia on the shoulder, former-general Shalal spoke from a desk in a room in an undisclosed location. Some rebel sources said he had fled to Turkey. It was not clear when the former army police chief had changed sides.

"The army has destroyed cities and villages and has committed massacres against an unarmed population that took to the streets to demand freedom," he said. "Long live free Syria."

Rebels 'gaining ground'

The defection came as opposition fighters continued to gain ground in battles against regime forces across the country. On Tuesday and after weeks of fighting, the fighters won control of a town near the Turkish border

The former major general was speaking from the Turkish-Syrian border, and said he was planning to defect but was waiting for the right circumstances to do so. "Definitely, there are other high-ranking officers who want to defect but the situation is not suitable for them to declare defection," he said, adding that the government is monitoring their movements.

The opposition Free Syrian Army (FSA) was in control of large swathes of Syrian territories, according to the former major general, warning that the Assad army, if further threatened, could use chemical weapons.

Following a reported government shelling in a village in Raqqa, the UK Syrian Observatory for Human Rights released video footage apparently showing the bodies of victims - 20 people, including eight children. However the fatalities have not been independently verified.

Opposition groups say more than 44,000 people have died in Syria since the conflict began 21 months ago.

More on this story