More than 100 combatants were killed over the last two days in northern Syria in fighting between Turkish-backed groups and Syrian Kurdish forces.
Since Friday evening, clashes in several villages around the city of Manbij have left 101 dead, including 85 members of pro-Turkish Syrian National Army (SNA) and 16 from the Kurdish-dominated Syrian Democratic Forces (SDF), the Syrian Observatory for Human Rights said on Sunday.
In a statement, the SDF said it had repelled "all the attacks from Turkey's mercenaries supported by Turkish drones and aviation".
SNA factions in northern Syria resumed their fight with the SDF at the same time Islamist-led rebels were launching an offensive on 27 November that overthrew Syrian president Bashar al-Assad just 11 days later.
They succeeded in capturing the cities of Manbij and Tal Rifaat in northern Aleppo province from the SDF.
The fighting has continued since, with heavy casualties.
According to Rami Abdel Rahman, the head of the Observatory, the Turkish-backed groups aim to take the cities of Kobane and Tabqa, before moving on to Raqqa.
The SDF controls vast areas of Syria's northeast and parts of Deir az-Zour province in the east where the Kurds created an autonomous administration following the withdrawal of government forces during the civil war that began in 2011.
The group, which receives US backing, took control of much of its current territory, including Raqqa, after capturing it from the militants of the Islamic State group.
Ankara considers the SDF an extension of the Kurdistan Workers Party (PKK), which has fought a decades-long insurgency in southeastern Turkey and is banned as a terrorist organisation by the government.
The Turkish military regularly launches strikes against Kurdish fighters in Syria and neighbouring Iraq, accusing them of being PKK-linked.
Ahmed al-Sharaa, Syria's new leader and the head of the Islamist group Hayat Tahrir al-Sham (HTS), has previously said the SDF would be integrated into the country's future army.
HTS led the coalition of rebel groups that overthrew Assad last month.