By Ayla Jean Yackley

ISTANBUL (Reuters) - Militants of the Kurdistan Workers Party (PKK) wanted a return to talks with the Turkish government a few months ago amid the worst violence in decades but Ankara rejected the overture, an opposition party leader said on Wednesday.

A 2-1/2-year peace process between the outlawed PKK and the government broke down last July and fighting since then has claimed thousands of lives, including around 500 civilians.

Chairman Selahattin Demirtas said his Peoples' Democratic Party (HDP), which has Kurdish origins, had persuaded PKK leaders based in the Qandil mountains of northern Iraq to return to the negotiating table.

"A few months ago, we were in contact with Qandil in an effort to return to the negotiating table. The government knew that we were working for this, but the government rejected it," Demirtas told a news conference in Istanbul.

"Qandil said it was ready for talks, but Ankara said there would be absolutely no return to the table. As long as it says this, can there be a one-sided negotiating table?"

President Tayyip Erdogan, who spearheaded the historic peace process before it collapsed last year, has publicly ruled out a return to negotiations and has vowed to crush the PKK, which has been waging a fight for greater autonomy since 1984 in a conflict that has killed more than 40,000 people.

Erdogan accuses the HDP of being the political wing of the PKK, which Demirtas denies.

Broadcaster CNN Turk, quoting the armed forces, said on Wednesday Turkish warplanes had struck PKK targets in northern Iraq. The Turkish military frequently strikes camps in Qandil.

Separately, the military said it had concluded operations in the town of Yuksekova on Wednesday, killing 196 PKK members since mid-March. Turkey, along with the United States and the European Union, considers the PKK a terrorist group.

'CRIMES'

Demirtas said the loss of civilian life and the brutality of security operations in southeast Turkey violated international conventions on war crimes, including collective punishment.

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His party is collecting evidence it hopes one day can be used to prosecute Erdogan, Prime Minister Ahmet Davutoglu and other officials, including the heads of the army and police.

The government denies any targeting of non-combatants and says the deaths of almost 400 soldiers and police since July are due in part to their efforts to minimize the loss of civilian life. It blames the deaths of civilians on the PKK for staging its fight in urban centers.

Demirtas cited eyewitness accounts including the killing of wounded civilians and destruction of homes. He said the HDP had been unable so far to find a prosecutor "brave enough" to investigate the allegations.

"Everyone responsible for this massacre will absolutely be tried one day. They need to know they cannot escape this. The president continues to give the same orders (such as) shelling civilian areas by the armed forces, which is a direct violation of the Geneva convention and a crime against humanity."

A senior government official told Reuters the charges were "groundless" and said such assertions from the HDP have little value because of its "support for a terrorist organization that has caused civilian deaths".

(Additional reporting by Orhan Coskun; Writing by David Dolan and Ayla Jean Yackley; Editing by Nick Tattersall and Gareth Jones)