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The defection of Syria’s military police chief represented another setback for the Assad government and came as military pressure builds on the regime, with government bases falling to rebel assault near Damascus and elsewhere across the country.

Dozens of generals, along with thousands of ordinary soldiers, have defected since Syria’s crisis began in March 2011. In July, Brig. Gen. Manaf Tlass became the first member of Mr. Assad’s inner circle to break ranks and join the opposition during the uprising, which anti-regime activists estimate has led to more than 40,000 deaths.

On Wednesday, the Britain-based Syrian Observatory for Human Rights said government shelling in the northeastern province of Raqqa had killed at least 20 people, including eight children and three women.

Also, activists said rebels were attacking the Wadi Deif military base in the northern province of Idlib. The base, which is near the strategic town of Maaret al-Numan, has been under siege for weeks.