GAZIANTEP, Turkey — The Syrian affiliate of Al Qaeda has announced its withdrawal from front-line positions against the Islamic State extremist group in northern Syria, saying that it disagrees with plans by Turkey and the United States to clear the extremists from an area along the Turkish border.

In a statement on Monday, the Qaeda group, the Nusra Front, said the proposed plan was intended primarily to protect “Turkish national security” and not to advance the Syrian rebel cause.

Syrian activists in the area reported the withdrawal of the Nusra Front in recent days, saying that other rebel groups had taken up their vacated positions to prevent an advance by Islamic State forces.

But on Monday, a Defense Department official said the United States did not believe the statement. “We’ve not yet seen any movements on the ground that would indicate they are following through with it,” the official said, speaking on the condition of anonymity to discuss battlefield issues.