Syria's foreign ministry said on Wednesday that it would consider any Turkish military incursions into its territory as "war crimes and crimes against humanity."
Turkish President Tayyip Erdogan on Monday said Ankara would soon create safe zones 30 km (20 miles) beyond its southern borders to combat what he characterized as terrorist threats, in a likely reference to Kurdish armed groups in northern Syria.
Ankara has already conducted three incursions into northern Syria since 2016, mainly targeting the US-backed Syrian Kurdish YPG.
Damascus sees the incursions as a violation of the country's sovereignty and territorial integrity.
On Wednesday, Syria's foreign ministry said it had sent a letter to the United Nations secretary-general and the Security Council, describing Turkey's actions as illegitimate.
"They amount to what can be described as war crimes and crimes against humanity," it said in a statement carried by the state news agency.