Syria's embattled leadership suffered a new setback Wednesday with the publicly broadcast defection of its military police chief, the highest-ranking officer to abandon President Bashar Assad since the uprising against him began nearly two years ago.

The defector, Maj. Gen. Abdul Azia Jassem al-Shallal, announced his move in a video broadcast by Al Arabiya, saying he had taken the step because of what he called the Syrian military's deviation from "its fundamental mission to protect the nation and transformation into gangs of killing and destruction."

Al Arabiya, a Saudi-owned pan-Arab broadcaster heavily critical of the Syrian government, said Al-Shallal had made the video Tuesday somewhere on the Turkish-Syrian border, implying that he was now inside Turkey, where other Syrian military defectors have sought refuge. Many have regrouped there to join the Free Syrian Army, the main insurgent force fighting Assad.

Reading from a prepared statement while sitting at a desk, dressed in a camouflage uniform with red epaulets, the general did not specify in his message when he had decided to defect but said that he had been "waiting for the right circumstances to do so."

He also said "there are other high-ranking officers who want to defect, but the situation is not suitable for them to declare defection."

Al-Shallal's statement came as Syrian insurgents were claiming new territorial gains against Assad in the northern and central parts of the country and as a special envoy from the United Nations and the Arab League was visiting Damascus as part of an effort to reach a political settlement that would halt the conflict, the most violent of the Arab Spring revolutions that began in the winter of 2010-2011. More than 40,000 people have been killed since protests against Assad began in March 2011.

There has been speculation that the special envoy, Lakhdar Brahimi, presented Assad with proposals for relinquishing his authority and possibly leaving the country. But Assad, whose Alawite minority has ruled Syria for more than four decades, has consistently said he will not leave the country, even as his control over it seems to be slipping further away.

Dozens of lower-ranking Syrian military officers and hundreds of soldiers have fled Syria over the past two years, but Al-Shallal, the head of the military police division of the Syrian army, is the highest-ranking military defector so far. He outranked Brig. Gen. Manaf Tlass, a boyhood friend of Assad's, who fled last July. Tlass is now believed to be living in France.

Among civilians who have abandoned Assad, the highest-ranking defector so far has been the prime minister, Riad Farid Hijab, who fled to Jordan on Aug. 6.