Turkish troops and Syrian opposition groups have reportedly entered the Islamic State (Isis) stronghold of Al-Bab on Saturday (11 February).
The British-based Syrian Observatory for Human Rights said allied soldiers from both groups entered from the western edge of the town where heavy clashes ensued.
Al-Bab is the last town held by IS (Daesh) in Aleppo province, after the Syrian government recaptured the city of Aleppo in December 2016.
The confrontation in Al-Bab has the potential to endanger the ongoing ceasefire between opposition groups and Bashar Al-Assad's government, with both sides vying for the territory.
On Monday (6 February), the Syrian Arab Army (SAA) encircled Al-Bab as it prepared to launch its own offensive on the town.
However, on Friday (10 February), before the SAA was able to proceed, it became involved in an exchange with Syrian militants on the outskirts of Al-Bab, 1.5km south-west of the town, until Russian officials intervened and halted the situation.
A Syrian opposition member told Reuters: "[We] shot to warn them not to get any closer, but the tank responded and a clash erupted."
"Later on Russia intervened to calm down the situation," added the militant official. "This whole incident felt like a test."
Russian officials declined to comment on the flare-up, but an official from the SAA confirmed the incident on the condition of anonymity.
Russia and Turkey, in addition to Iran, have acted as guarantors to the Syrian ceasefire reached in December and extended in the Kazakh capital of Astana in January.
Further talks are scheduled there during 15-16 February, and talks backed by the United Nations are due to take place in Geneva on 20 February.
Turkey entered the Syrian conflict with operation Euphrates Shield in August while Syrian forces focused on the capture of Aleppo.
They and anti-government militants have since created a buffer zone in the north of the country where they launched offensives against IS and Kurdish forces such as the People's Protection Units (YPG) and the Kurdish-led Syrian Democratic Forces (SDF).
Turkey sees the YPG as an extension of the Kurdistan Workers Party (PKK) – a militant group within Turkey which has been fighting for its autonomy since the 1980s.