More than 70 Syrian military officers, including six generals and 22 colonels, have deserted in the past 36 hours and have crossed the border into Turkey, the biggest such defection in months, a Turkish official told AFP on Saturday.
The defections of the 71 army officers and two policemen came after Washington said it had proof that Syrian government forces had carried out deadly chemical weapons attacks against rebels and the United States would begin providing military support to the opposition.
Observers in Ankara said the defections signalled that the US policy shift could have a serious impact on President Bashar-al Assad's army.
US President Barack Obama's administration announced late on Thursday that it had reviewed intelligence reports and concluded that Syrian regime forces had used banned weapons, including the nerve agent sarin, in attacks that killed up to 150 people.
US officials refused to rule out moving towards arming rebels or imposing a no-fly zone, and said Washington would provide backing to the rebel Syrian Military Council.
Britain and France, which had already said publicly that they believed the Syrian government had used chemical weapons, welcomed the US announcement.
Damascus has rejected the US accusations as "lies", while Moscow, a key Assad ally, said the allegations were "unconvincing" and hurt efforts to make peace.
Turkey, a one-time ally of Damascus, has become one of the fiercest critics of Assad's regime and is a vocal supporter of the rebels fighting to overthrow him.
Since the start of the civil war in Syria in March 2011, dozens of senior army officers have defected to Turkey, though Turkish authorities have refused to give an official estimate.
More than 90,000 people have died in the Syrian conflict so far and Turkey is hosting some 400,000 refugees.
Representatives from countries that support the Syrian opposition are currently meeting in Istanbul with rebel military chief General Selim Idriss to discuss the possible delivery of weapons.