The Turkish air force has struck pro-government Syrian fighters deployed last week in the Kurdish enclave of Afrin, killing and wounding a number of them.
The incident, which targeted two positions of the pro-Syrian force, marks an escalation of violence in a region where Turkey has been on the offensive for more than a month.
Turkish troops and Turkey-backed opposition fighters have operated in Afrin since January 20 against Syrian Kurdish fighters. Ankara has also warned Damascus not to send fighters to the area, saying that it would target them.
Turkey considers Syrian Kurdish fighters to be "terrorists" linked to a Kurdish insurgency inside its own borders.
The pro-government Syrian force began deploying in Afrin on Feb 20, despite Turkey's threats. Syrian state media said the aim is to defend Afrin.
The air strikes mark a major escalation between Turkey, the main backer of Syria's opposition fighters trying to remove President Bashar Assad from power, and Syrian pro-government forces, backed by Iran.
The Syrian Observatory for Human Rights said the air strikes took place late on Thursday in the village of Jamaa and killed 17 fighters of the force known as the Popular Forces.
The Syrian state news agency said Turkish forces killed 20 people in Afrin. The Lebanon-based Al-Mayadeen TV said Turkish warplanes had killed 18 pro-government gunmen, while 19 more are missing.
Al-Mayadeen said the strike occurred after the Popular Forces hit a Turkish helicopter gunship.
The main Kurdish militia in Syria, the People's Protection Units, or YPG, confirmed the attack, saying the air strikes killed and wounded several fighters.
Turkey's military said Turkish-made ATAK helicopters struck a region in western Afrin, killing nine "terrorists". It is not clear if the air strikes were in retaliation for the deaths of eight Turkish soldiers who were reported killed there on Thursday.
Near the capital, Damascus, a five-hour truce went into effect on Friday, the fourth this week, with no civilians leaving the city's rebel-held suburbs known as eastern Ghouta.
State-run al-Ikhbariya TV said rebels shelled a crossing point to Ghouta in order to prevent people from leaving.
The Russia-ordered pause came after a UN Security Council resolution calling for a nationwide 30-day ceasefire failed to take hold. While the relentless bombing has somewhat subsided in eastern Ghouta, home to around 400,000 civilians, the Syrian government's push to squeeze the insurgents out of the region continues.
The Observatory and the opposition's Syrian Civil Defenxe, also known as White Helmets, reported air strikes earlier on Friday on the suburbs of Zamalka, Douma and Hazzeh. The Syrian Civil Defence said one person was killed.
- Press Association