Kurdish militants killed 10 soldiers and wounded at least 60 when they fired rockets at a military convoy in eastern Turkey on Tuesday, security sources said.
The past few months have seen some of the heaviest fighting since the PKK, considered a terrorist organization by Turkey, the United States and the European Union, took up arms in 1984 with the aim of carving out a Kurdish state.
The army convoy was travelling between the provinces of Bingol and Mus in eastern Turkey when it was hit by a rocket launched by PKK fighters, security sources said.
Footage aired by Turkey's Dogan news agency showed firefighters around the smoldering shell of a bus on a mountainous road.
Security sources said a military operation had been launched in the area after the attack and a Reuters witness saw at least two F-16 fighter jets take off from an air base in the southeastern city of Diyarbakir. Their destination was unclear.
Fighter jets and attack helicopters have been bombarding PKK camps in the southeastern border region with Iraq, and Turkey's chief of staff and other military commanders travelled to the region last week to oversee operations.
More than 40,000 people have been killed in decades of conflict. Prime Minister Tayyip Erdogan said on Monday some 500 militants had been "rendered ineffective" - killed, wounded, or captured - in the last month alone.
Eight police officers were killed on Sunday when a roadside mine exploded in the Karliova district of Bingol. On Saturday suspected PKK fighters killed four Turkish soldiers in an attack on a convoy near the border with Iran and Iraq.
As well as the human toll, the conflict has hampered economic development in one of Turkey's poorest corners and worsened instability in a region bordering Iran, Iraq and Syria.
Some Kurdish politicians have called for an immediate ceasefire on both sides as a first step to ending the fighting. But, faced with a rising death toll among its security forces and broad public support for a harsh military response, Ankara has shown no inclination over the past year to make concessions.
Turkey has said it will stop conducting military operations only if PKK militants lay down their arms first. Hopes of an end to conflict rose after Erdogan's clear victory in elections last year, when he vowed to press ahead with cultural and political reforms for Kurds, reversing assimilation policies that bred Kurdish resentment.
Kurds make up roughly a fifth of Turkey's population but for decades an avowedly nationalist state refused to recognise their existence and banned their language and culture.
The EU ban on the PKK has made it harder for the Kurds in western Europe's large Turkish diaspora to provide it with support. Danish police said that an investigation of Turkish citizens of Kurdish origin had yielded the arrests of eight people suspected of funneling around $25 million to the PKK since 2009.
In January, a Danish court found the owners of the Kurdish television station Roj TV, an international satellite channel based in Denmark, guilty of promoting terrorism and fined them 5.2 million crowns.