BERLIN: Germany's spy chief said Syria President Bashar Assad's government appeared to be in its final phase because its army had been depleted by casualties, deserters and defectors to the opposition.

Gerhard Schindler, head of Germany's BND intelligence agency, said Assad's once 320,000-strong army had lost about 50,000 troops since the uprising against his rule began 17 months ago.

Smaller, flexible rebel units were sapping the strength of the army with guerrilla tactics, he told Die Welt newspaper in an interview published on Saturday.

"There are a lot of indications that the end game for the regime has begun," said the president of the Bundesnachrichtendienst agency.

"That (army losses) includes those who have been wounded, deserted and about 2,000 to 3,000 who have defected to the armed military opposition," he said. "The erosion of the military is continuing."

While Assad's grip on the country has been loosened as the uprising has gathered momentum, his forces have overwhelming firepower advantage against lightly armed rebels.

However, Schindler said small rebel units were offsetting that by using their speed and manoeuvrability to strike quickly in ambushes.

"Because of their small size, they're not a good target for Assad's army," he said. "The regular army is being confronted by a variety of flexible fighters. The recipe of their success is their guerrilla tactics. They're breaking the army's back."

Assad is fighting to crush a rebellion that aims to end his family's four decades in charge of Syria.