Turkey's President Recep Tayyip Erdogan on Monday said Ankara would soon create what he called "safe zones" 30 kilometres beyond its southern borders to combat "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.
The group is believed to be linked to the banned Kurdistan Workers' Party (PKK) which has been seeking Kurdish rights in Turkey.
Damascus sees the incursions as a violation of Syria's sovereignty and territorial integrity, though the areas Turkey currently has its eyes on are largely held by an autonomous Kurdish-led administration.
On Wednesday, the Syrian 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 regime's news agency.
The Syrian war erupted in 2011. Since then, the war has killed 500,000 people.
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