The head of Syria's military police has defected from the army and declared allegiance to the uprising against President Bashar al-Assad.

Major General Abdelaziz Jassim al-Shalal was shown making a statement confirming his defection in a video broadcast on al-Arabiya TV late on Tuesday, saying he was joining "the people's revolution".

The defection came as a delegation of Syrian officials headed to Moscow on Wednesday to discuss proposals for ending the conflict following talks with the UN envoy Lakhdar Brahimi in Damascus this week.

Wearing his uniform with a red insignia on the shoulder, 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 Shalal 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."

The defection will be a blow to morale for Assad's forces, which are hitting back at a string of rebel advances across the country. It follows the defections of dozens of other generals since Syria's crisis began in March 2011.

In July Brigadier General Manaf Tlass was the first member of Assad's inner circle to break ranks and join the opposition. Shalal, however, is one of the most senior, and held a top post at the time he left.

In his statement he said the army had been "derailed from its basic mission of protecting the people and … become a gang for killing and destruction".

He accused the military of "destroying cities and villages and committing massacres against our innocent people who came out to demand freedom".

Thousands of Syrian soldiers have defected over the past 21 months and many of them are now fighting against government forces. Many have cited attacks on civilians as the reason they switched sides.

A Syrian security source confirmed Shalal's 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.

A group of Syrian foreign ministry officials headed to Moscow to discuss proposals apparently made by Brahimi. The deputy foreign minister, Faisal Makdad, and an aide will sound out Russian officials on the details of meetings with Brahimi, a Syrian security source said.

A Lebanese official close to Assad's government said Syrian officials were upbeat after talks with the UN-Arab League envoy, who met the Syrian foreign minister, Walid Moualem, on Tuesday and Assad himself the day before.

"There is a new mood now and something good is happening," the official said, asking not to be named due to the sensitivity of the issue. "Of course now they [Syrian officials] want to meet with their allies to discuss these new developments."

More than 44,000 Syrians have died in the revolt against four decades of Assad family rule, a conflict that began with peaceful protests and which has descended into civil war.

Brahimi is in Syria for a week of talks with government officials and some dissidents, but has so far said nothing about any new proposals or developments.

Earlier in December he held tripartite meetings between Russia, Syria's main arms supplier and an Assad ally, and the United States, which has thrown its weight behind the opposition. While both sides said they wanted a political settlement, neither changed their stance on Assad.

Brahimi's previous proposal centred on a transitional government which left open Assad's future role, something which became a sticking point between the government, the opposition and foreign powers backing different sides.

The latest moves emerged as a video posted on Wednesday claimed government shelling in the northern Syrian province of Raqqa had killed about 20 people, at least eight of them children,.

The Syrian Observatory for Human Rights published a video showing rows of the blood-stained bodies laid out on blankets. The sound of crying relatives could be heard in the background. It was unclear when the attack in the village of al-Qahtania happened.