Turkey has arrested more than 700 people it suspects of links to the Kurdistan Workers' Party (PKK), after it accused the group of executing 13 Turkish nationals.
The victims - soldiers, police and civilians who were allegedly abducted by the insurgents over the last few years - were discovered in a cave complex in the Gara region in Iraq, near the Turkish border.
They were found during an operation against the PKK, launched on February 10, that had aimed to free the hostages.
Twelve of the victims were shot in the head and one died of a shoulder bullet wound, the Turkish defense minister said on Sunday.
On Monday Turkish president Recep Tayyip Erdogan accused the United States of supporting Kurdish militants, calling the statement from the US “deplorable”.
“You say you don't support the terrorists but you are actually on their side," Erdogan said in a speech.
The US State Department had said on Sunday it "deplores" the deaths.
"If the reports of the deaths of Turkish civilians at the hands of the PKK, a classified terrorist organisation, are confirmed, we condemn these actions in the strongest terms," it added in a statement.
The PKK has waged an insurgency against the Turksih state since 1984, with tens of thousands of people killed in the fighting.
It is designated a terrorist organisation by the US and the EU. However US support for Syrian Kudish militias of the YPG, which began under the Obama administration, has for several years caused tensions between the US and Turkey, the latter claiming the YPG is linked to the PKK.
Turkey and US clash over support for Kurdish militias
“Did you not say you don’t support the PKK, the YPG or the PYD? You are with them and behind them pure and simple,” Erdogan said.
“If we are together in NATO, and if we are to continue our (alliance) in NATO, you have to be sincere toward us,” Erdogan said. “You must not take the side of the terrorists. You have to be on our side.”
Erdogan said 51 PKK militants were killed during the offensive and vowed to press ahead with cross-border offensives against the PKK in Iraq, and against the Syria-based militants.
In a statement carried by the PKK-linked Firat news agency, the PKK said “prisoners of war” consisting of members of the Turkish security forces and intelligence agency were killed as a result of Turkish air strikes.
Three Turkish troops also died during the operation to free the hostages and three others were wounded, the defense ministry has said.
The Turkish Interior Ministry announced on Monday that security forces had arrested 718 people, including HDP leaders, on suspicion of links with the PKK.
"A large number of weapons, documents and digital equipment belonging to the (terrorist) organisation were seized during the searches," the interior ministry added, adding that operations in 40 cities across the country were still under way.
The HDP had on Sunday expressed its "deep sadness" after the death of Turkish nationals in Iraq, calling on the PKK to release its remaining prisoners.