The alleged killings threaten to heighten tensions between Ankara and Baghdad, as well as with Washington, which backs groups tied to the PKK in Syria.

Ankara has long accused the Iraqi government of being too tolerant of the PKK, listed as a terror group by Turkey and much of the international community.

The PKK has for decades used Iraq's mountainous areas as a springboard for its insurgency against the Turkish state.

Defence Minister Hulusi Akar said Turkish soldiers had discovered 13 bodies in a cave in the Gara region of northern Iraq, where Ankara launched an operation against the PKK on Wednesday.

Each of the victims was shot dead with one bullet to the head or chest soon after Turkish soldiers launched an assault on the cave, Akar said, citing the testimony of two Kurdish fighters who were taken prisoner.

The governor of eastern Turkey's Malatya province, where the bodies were taken, said 10 of the victims have been identified. Most were soldiers and police officers who were kidnapped by the PKK in 2015 and 2016.

Citing autopsy reports, the governor, Aydin Barus, said the victims appeared to have been shot at pointblank range.

The PKK on Sunday admitted that a group of prisoners had died but rejected Ankara's version of events, saying instead that they had been killed by Turkish air strikes.

AFP could not independently confirm either version.

Meanwhile Defence Minister Akar said 48 PKK fighters and three Turkish soldiers had been killed in northern Iraq since Wednesday.

The Turkish army regularly conducts cross-border operations and air raids on PKK bases in northern Iraq.

The operations have strained relations with Baghdad, but Turkish President Recep Tayyip Erdogan has repeatedly said his country would "deal with" the PKK in northern Iraq if Baghdad did not.

The Kurdish insurgency against the Turkish state is believed to have killed tens of thousands of people since being launched in 1984.

In December, Erdogan called on Iraq to step up its fight against the PKK during a visit to Ankara by Iraqi PM Mustafa al-Kadhimi.

The losses reported Sunday threaten to heap pressure on Turkey's pro-Kurdish Peoples' Democratic Party (HDP) party, which Erdogan accuses of being a political front for the outlawed PKK.

The HDP is Turkey's second-largest opposition group in the parliament.

Dozens of HDP elected and party officials have been arrested since 2016, raising concerns among Western countries.

On Sunday the HDP expressed "deep sadness" over the deaths of the 13 Turks in Iraq, calling on the PKK to free its remaining prisoners.