Turkey summoned the Russian charge d'affaires on Wednesday and sent him another message Thursday to convey "deep unease" over two incidents in Kurdish militia-controlled Syria, Ankara said.
Ankara said one of its soldiers had been shot and killed in southern Turkey on Wednesday by cross-border sniper fire from an area of northwestern Syria controlled by the Kurdish YPG militia.
"Immediately after the incident, we invited the Russian charges d'affaires to the ministry and we first shared how seriously disturbed we were," Turkish foreign ministry spokesman Huseyin Muftuoglu told reporters in Ankara.
Under a ceasefire agreed late last year, Russia is responsible for monitoring violations in this region, he explained, saying that Ankara had warned that it would respond in kind if the incident was repeated.
On Thursday, the foreign ministry sent a new message to the Russian envoy over photos of Russian troops apparently showing them wearing YPG insignia, state-run news agency Anadolu said.
"The Russian party is familiar with our position" on YPG, diplomatic sources told Anadolu. "Therefore, those images have inconvenienced us. That was the message we communicated."
A ceasefire brokered by Russia and Turkey has been in place since late December but both Syrian rebels and the Damascus regime have complained of repeated violations.
Earlier this week, YPG spokesman Redur Xelil told AFP that Russian troops would train Kurdish forces in Syria, saying the Russians were already present at a training camp in the Afrin region.
Afrin is one of three "autonomous" cantons controlled by the Kurdish militia in northern Syria.
Ankara views the Kurdish People's Protection Units (YPG) militia as a "terror group" linked to Kurdish separatists waging an insurgency inside Turkey since 1984.
Muftuoglu also said Turkey was waiting for Russia to "take steps to close" the Moscow office opened last month of the Kurdish Democratic Union Party (PYD), whose armed wing is the YPG.
Moscow supports Syrian President Bashar al-Assad but has also recently worked closely with opposition supporter Turkey to try to end the six-year war in Syria.