The Turkish offensive began on Wednesday (Picture: PA).
The NATO Secretary General says Turkey is at "risk of further destabilising" Syria if it continues its offensive in the country.
Jens Stoltenberg arrived in Turkey on Friday and said he had "serious concerns" about the ongoing operation in northern Syria.
He said he had raised these with the country's minister, Mevlüt Çavuşoğlu:
"I shared with him my serious concerns about the ongoing operation and the risk of further destabilising the region, escalating tensions and even more human suffering."
The operation, named 'Peace Spring', began earlier this week when Turkish forces stepped up their cross-border offensive on Kurdish-held areas.
It follows the removal of some US troops, who fought alongside Kurdish forces in north-east Syria against so-called Islamic State (IS).
WATCH: Sian Grzeszczyk reports on the situation in Syria.
Mr Stoltenberg said Turkey, a NATO member nation, needed to be cautious.
"While Turkey has legitimate security concerns, I expect Turkey to act with restraint," he said.
Mr Stoltenberg spoke of the progress NATO allies had made in liberating areas in Iraq and Syria but warned: "These gains must not be jeopardised."
He further raised the risk of detained terrorists, adding: "An imminent concern is that captured Daesh terrorists must not be allowed to escape."
Since the launch of the countries offensive, there have been casualties on both sides and Turkey reported its first military fatality, saying a soldier was "martyred" in the fighting and three others were injured.
At least six civilians are reported to have been killed in Turkey and seven more in Syria since Ankara launched its air and ground operation, a move it said is necessary to protect national security.
The Turkish defence ministry also said 49 more "terrorists" had been "neutralised" in the incursion, in reference to Syrian Kurdish fighters, saying it has now killed a total of 277.