Turkish air strikes killed at least 17 Syrian pro-government fighters in Afrin on Thursday night, activists said, hours after eight Turkish soldiers were killed in the region.
The Britain-based Syrian Observatory for Human Rights said the air strikes hit the village of Jamaa and killed 17 fighters of the Popular Forces, who were deployed in the Kurdish enclave in north-western Syria last week.
Turkey's military said helicopters struck a region in western Afrin, killing nine "terrorists". It did not provide further details and it was not clear if the air strikes were in retaliation for the deaths of the Turkish soldiers.
The Turkish military staff released two separate statements earlier announcing that eight soldiers were killed and 13 others wounded in Afrin, where it began a cross-border operation against the People's Protection Units (YPG) Kurdish militia on January 20.
"As part of the operations in Afrin, five of our heroic comrades fell as martyrs and seven were wounded" on Thursday, the first statement said.
Shortly after, a second statement announced that three more soldiers had been killed as well as six wounded, without giving details of the circumstances.
The private Dogan news agency reported that intense fighting had broken out in the afternoon between Turkish special forces units recently deployed in Afrin and YPG members, who mounted an ambush with the help of tunnels.
A Turkish helicopter sent to rescue the wounded had to turn back after being hit, while the area was shelled to allow an evacuation.
Turkish President Recep Tayyip Erdogan was informed of the incident during a visit to Senegal.
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Ankara considers the YPG a terrorist organisation closely linked to the Kurdistan Workers' Party (PKK), a group that has been leading a bloody guerrilla war on Turkish soil since 1984.
However, the YPG is supported by the United States and has been spearheading the international coalition fight against the ISIL in Syria.
The situation was complicated further 10 days ago following the deployment of pro-regime elements in the enclave of Afrin, with observers warning of an increased risk of confrontation between the forces of Ankara and Damascus.
On Monday, Turkey deployed about 600 members of the police and gendarmerie special forces in the Afrin region, indicating it was preparing for urban fighting.
The Turkish authorities rejected a call by the United States this week to implement the humanitarian truce in Syria called for by the UN Security Council, with Ankara saying the UN resolution did not concern its operation.
The International Committee of the Red Cross said a humanitarian aid convoy entered the Afrin region on Thursday for the first time since the start of the Turkish offensive, which has had a severe impact on civilians.
According to the Syrian Observatory for Human Rights, 141 civilians, including 27 children, have died since the beginning of the Turkish military campaign, a claim which Ankara denies.
The number of Turkish soldiers killed since the launch of "Operation Olive Branch" in Afrin has risen to at least 40.