AMMAN (Reuters) - Syria demanded on Saturday that Turkish troops who had entered northwestern Syria leave immediately, saying their presence constituted a "flagrant aggression".

A Foreign Ministry statement carried on state media said the entry of Turkish military vehicles into rebel-held northwest Syria through the Bab al-Hawa crossing late on Thursday was a violation of international law.

"Syria condemns in the strongest terms the incursion of units of the Turkish army in Idlib province, which constitutes a flagrant aggression against the sovereignty and security of Syrian territory," the statement said.

Turkey says it is operating alongside Syrian rebel groups to implement a deal reached last month with Russia and Iran in the Kazakh capital Astana to reduce fighting between insurgents.

But Syria's statement said: "The Turkish aggression is not tied in any way with the understandings that were reached between the guarantor states in the Astana process, but constitutes a violation of these understandings."

(Reporting by Suleiman Al-Khalidi; Editing by Kevin Liffey)