By Ayla Jean Yackley
DIYARBAKIR, Turkey (Reuters) - In a public cemetery next to a military air base in Turkey's mainly Kurdish southeast, flags of a Syrian Kurdish militia are draped over many of the tombstones.
Death notices posted online by the People's Protection Units (YPG), a key U.S. ally in the fight against Islamic State in north Syria, show about half of those killed on its front lines in the last three months alone were Turkish-born.
Sertip Celik, a student in the Mediterranean town of Iskenderun, was one of thousands of Turkish Kurds to cross into Syria and join the fight against Islamic State, answering a call to arms by the outlawed Kurdistan Workers Party (PKK), which has waged an insurgency against Turkey for three decades.
"What was being done to Kurds in Syria and Iraq, the attacks and the massacres, the kidnapping of women as slaves, repulsed him," said his mother, Rahime Celik. "No mother wants her son to go to his death, but I could not have stopped him."
Within three months of leaving, Sertip was dead, killed during the 2014 Islamic State siege of Kobani just over the border from Turkey.
Solidarity with their ethnic kin in Syria is a source of pride for many of Turkey's Kurds.
But Ankara sees the links between the YPG and PKK as a threat to its unity and security, complicating international efforts to end Syria's war and creating tensions with its NATO ally the United States. It fears the creation of a Kurdish fiefdom in Syria will fuel separatist ambitions at home.
Washington, like Turkey and the European Union, lists the PKK as a terrorist organization. But much to the frustration of Ankara, it sees the YPG and its political arm, the Democratic Union Party (PYD), as a distinct group, with which it is willing to work to battle Islamic State.
"In its own reports, the U.S. has described the PYD, with all of its elements, as a terrorist group that procures its weapons from the PKK," Saadet Oruc, a senior adviser to Turkish President Tayyip Erdogan, told Reuters.
"To not see this link now requires you to close your eyes and bury your head in the sand. Statements from the U.S. that they see these as different groups are just not credible."
Erdogan has said the YPG and the PKK jointly planned and carried out a suicide bombing in the heart of the capital Ankara last week that killed 29 people, most of them soldiers.
Prime Minister Ahmet Davutoglu has called the attack clear evidence of the terrorist nature of the YPG and said Ankara expected cooperation from its allies in combating the group.
The U.S. military, however, has found an adept partner in the YPG. The alliance, working with Iraqi Kurdish peshmerga fighters, has pushed Islamic State out of Kobani and other jihadist strongholds in Syria and Iraq since 2014.
That has allowed the PYD to keep control of three Syrian provinces along the Turkish border, which it runs autonomously under the name "Rojava", referring to the western end of Kurdish territories stretching across Iran, Iraq, Turkey and Syria.
Before the war in Syria, the Syrian Kurdish militia was part of the PKK's rank and file in northern Iraq.
The PKK founded the YPG as a Syrian organization a decade ago and both groups are inspired by Abdullah Ocalan, who led the PKK from inception and lived in Syria shortly before his capture in 1999, said Aliza Marcus, author of "Blood and Belief", a history of the guerrilla group.
But the Syrian outfit has taken pains to distinguish itself.
"The YPG is the Syrian arm of the PKK but the Syrian Kurds are operating based on their own needs and goals," Marcus said.
"The leadership is made up of Syrian Kurds, and they have a different outlook, focused on Rojava."
That alliance has stirred the ire of Erdogan, who has said Washington must choose between Turkey and the Kurds.
In the early stages of Syria's civil war, there were signs Turkey was willing to work with the PYD and other Kurdish groups if they met three demands: to remain resolutely opposed to President Bashar al-Assad, vow not to seek autonomy through violence and pose no threat to Turkey.
It hosted PYD leader Saleh Muslim for talks when it was negotiating with the PKK to end the conflict in southeast Turkey that has killed 40,000 people since 1984.
But a breakdown in the peace process in July and YPG advances in Syria, where the group has also taken advantage of a Russian-backed government offensive to seize territory from Turkish-backed rebels, has infuriated Ankara.
Gultan Kisanak, mayor of Diyarbakir, the largest city in Turkey's mainly Kurdish southeast, said a Turkey at war with Kurdish militants at home was left little choice.
"Had Turkey made peace with Kurds at home, provided them with democracy and stopped the clashes, it would not see the Syrian Kurds as a threat," she said in an interview.
"When the government's political aims in Syria failed and clashes began in Turkey, it began its campaign that the PYD and YPG were terrorists."
Few Kurds question the connection between the PKK and the YPG - echoing the Turkish government's assertion.
Celik's first brush with conflict came at the age of five, his mother said. Soldiers evacuated their village, worried it harbored PKK sympathizers, and the family fled to Diyarbakir. His father spent most of the 1990s in jail on political charges.
Like many of the Turkish Kurds fighting in Syria, Celik had no military training. He had been the first of the family's seven children to attend university. He rarely spoke of politics but promised his mother, "I will be a great man one day," she said.
Intelligence sources cited by Turkish newspapers last year said about 4,500 of the YPG's estimated fighting force of 30,000 were Turkish-born and another 4,000 came from PKK bases in northern Iraq, where the group has been based since the 1990s.
"Kurds may live within the borders of different states but they have mutual, cultural, emotional links," said Kisanak. "These borders have only been around for 100 years, and when they were drawn, families, villages and cities were divided. When one side suffers, the other is affected too."
At the Diyarbakir cemetery where Celik rests, two dozen of the freshest graves have yet to be encased in marble, and grieving families have spelt out the names of the dead in pebbles in the dirt.
Turkish jets that have been bombing PKK camps in northern Iraq roar overhead from the nearby airbase, where the Turkish national anthem sometimes rings out.
"I watch the funerals of soldiers and police on TV, and I recognize the pain of the mothers as they cry and scream. It is the souls of mothers that burn," said Rahime Celik. "I wish Sertip had never gone. I wish this endless war would be over."
(Editing by Nick Tattersall and Andrew Roche)