Seven Turkish soldiers have been shot dead after gunmen opened fire on a military unit in northern Turkey, officials have said.
A further four soldiers were injured in the attack, which took place in the town of Resadiye in Tokat province.
There was no immediate indication of who was behind the attack. However, both Kurdish and leftist militants have come under suspicion.
Attacks on military bases in the north of the country are, however, rare.
According to one local television report, the soldiers were on patrol near a military outpost in a mountainous area and during foggy weather when they came under fire.
It was the worst attack since April when Kurdish militants killed 10 soldiers with a remote-controlled bomb in the country's southeast.
The ambush came as Prime Minister Tayyip Erdogan was preparing to meet President Barack Obama in Washington. Government moves to improve Kurdish rights are likely to be on the agenda.
There have been rising ethnic tensions over recent days ahead of a court hearing on whether the country's largest pro-Kurdish political party, the Democratic Society Party, should be closed down.
On Sunday, one man died in clashes between police and protesters in the south-east of the country.
In the past, the outlawed Kurdistan Workers Party (PKK) has carried out attacks on military targets. Extreme leftist groups active in northern Turkey are believed to have signed co-operation pacts with the PKK.