"The French (intelligence) services helped me get out of Syria and I thank them for that," Tlass, whose July 6 defection was hailed in the West as a major setback for Assad, told France's BFM television news channel.
A general in the elite Republican Guard charged with protecting the regime, Tlass is the son of former defence minister Mustafa Tlass, a close friend of Assad's late father and predecessor, Hafez al-Assad.
His defection was welcomed by the opposition umbrella group the Syrian National Council as an "enormous blow" to Assad.
But he has faced criticism from Syrian rebels, who say he and his 80-year-old father, who lives in Paris, should have made their positions clear at the very start of the anti-Assad uprising in March last year.
But Manaf Tlass said Monday he had gone over to the rebel side "since the month of March", without specifying if he meant March this year or last year.
"Since the start of the revolution I had meetings with the revolutionaries ... and I had the feeling from the first days, the first months, that the regime was lying to everybody. That is why I at first defected while staying in my office," he said.
Tlass said he was opposed to foreign intervention in Syria but called on the international community to arm the rebels.
"We must support them, we must help them, we must arm them," he said.
Tlass also said it was a possibility that Assad's regime could one day use chemical weapons.
"The regime is capable of using all kinds of weapons. If it is cornered, it's possible it would use chemical weapons," he said.
