Turkey said it would retaliate after three of its soldiers were killed in what the military said was a suspected Syrian air strike, the first such deaths at the hands of Syrian government forces since Ankara launched a cross-border incursion in August.
The attack occurred at around 3:30 am (0030 GMT) on Thursday during a Turkish-backed Syrian rebel operation in northern Syria, the Turkish military said in a statement.
It said 10 other soldiers were wounded in the air strike that it "assessed to have been carried out by Syrian regime forces". It gave no details on the exact location.
"It is clear that some people are not happy with this battle Turkey has been fighting against Daesh (Islamic State). This attack will surely have a retaliation," Prime Minister Binali Yildirim told reporters in the capital Ankara.
Direct confrontation between NATO-member Turkey and Syrian government forces, which are backed by allies including Russia, would mark a serious escalation in an already messy battlefield in northern Syria.
Turkey is part of the US-led coalition against Islamic State, but Washington has said it is not providing support for the three-month-old Turkish offensive in Syria as it moves towards the Islamic State-held city of al-Bab.
There was no immediate comment from the Syrian military. But it said in October the presence of Turkish troops on Syrian soil was a "flagrant breach of Syria's sovereignty" and warned it would bring down Turkish warplanes entering its air space.
Security and hospital sources in Turkey earlier blamed Islamic State fighters for the attack and said it was in the al-Bab region. The wounded soldiers were transferred to hospitals in the Turkish border provinces of Kilis and Gaziantep, they said.
Turkey sent tanks, Special Forces and jets into Syria on Aug. 24 in support of largely Turkmen and Arab rebels in an offensive dubbed "Euphrates Shield" meant to push Islamic State and Kurdish militia fighters from its border.
President Tayyip Erdogan said last week that the Turkish-backed rebels were close to taking the Syrian city of al-Bab, the last urban stronghold of Islamic State in the northern Aleppo countryside.
Forces allied to Syrian President Bashar al-Assad warned Turkey last month against making any advance towards their positions to the north and east of Aleppo, saying any such move would be met "decisively and with force".
The Turkish-backed rebels have clashed with Syrian government forces before, including in late October, when a suspected Syrian government helicopter bombed their positions near Dabiq, a former Islamic State stronghold.
But the overnight clash was the first time the Turkish military has said its own soldiers were killed by Syrian forces since Euphrates Shield began.
The attack came on the first anniversary of Turkey shooting down a Russian warplane over Syria, which prompted a lengthy diplomatic rift between Moscow and Ankara which only ended in August. Moscow is a major military backer of Assad.