Syria's top diplomat has demanded US and Turkish forces immediately withdraw from the country and says his government reserves the right to defend its territory in any way necessary if they remain.
"The United States and Turkey maintain an illegal military presence in northern Syria," Foreign Minister Walid al-Moallem has told the UN general assembly.
"Any foreign forces operating in our territories without our authorisation are occupying forces and should withdraw immediately."
If they refuse, he said on Saturday, "we have the right to take any and all countermeasures authorised under international law".
There are around 1000 US troops in Syria on a mission to combat Islamic State group militants.
The US also backs and supports Kurdish groups in the northeast that are opposed to the Syrian government and have fought against Sunni extremist groups.
US President Donald Trump has said he wants to bring the troops home, but military officials have advocated a phased approach.
Al-Moallem described Turkey and the US as "arrogant to the point of holding discussions and reaching agreements on the creation of a so-called 'safe zone' inside Syria" as if it was on their own soil.
He said any agreement without the consent of the Syrian government is rejected.
The deal between Washington and Turkey details an area 5-14km deep, as well as removal of heavy weapons from a 20km-deep zone along Syria's northeast border with Turkey.
For more than eight years, Syria's devastating civil war has drawn numerous foreign militaries and thousands of foreign fighters battling for power.
Most of the country has now returned to government control. But rebels and extremists still hold Idlib in the northwest, and US-backed Kurdish groups hold the oil-rich northeast.
Australian Associated Press