* Erdogan says may have been internal feud
* Turkey puts European missions on alert
* Kurds demonstrate in Germany and Sweden
By Ayla Jean Yackley
ISTANBUL, Jan 11 (Reuters) - Kurdish rebels suggested on Friday that clandestine Turkish nationalists may have assassinated three Kurdish activists in Paris, but Prime Minister Tayyip Erdogan said the killings appeared to have been the result of an internal feud.
The Kurdistan Workers Party (PKK) said the execution-style killings of the three women in an institute in central Paris had been premeditated and planned and warned France would be held responsible if it failed to get to the bottom of their deaths.
Sakine Cansiz, a founding member of the PKK, and two fellow activists were found shot in the head early on Thursday in an attack which shocked the Kurdish community and overshadowed peace moves between Turkey and the rebels.
Turkey put its missions in Europe - home to a large Kurdish diaspora - on alert and asked the French authorities to boost security around its interests there, after the pro-Kurdish Peace and Democracy Party (BDP) called for protest meetings.
In Berlin, home to a large Kurdish and Turkish population, some 700 Kurds demonstrated in the streets, many carrying posters of the three women. One group carried a sign saying, "Women are murdered, Europe is silent."
About 200 people stood in sub-zero temperatures outside the French embassy in Stockholm chanting "Long Live the PKK" and "Turkey, Terrorists."
"The targeting of three of our female comrades at a time like this is a premeditated, planned and organised attack," said a statement on the website of the armed wing of the PKK, deemed a terrorist group by Ankara, Washington and the European Union.
"France has a responsibility to elucidate these killings immediately. Otherwise, they will be held responsible for the massacre of our comrades."
The statement blamed "international and Turkish Gladio forces" for the killings, a reference to NATO's Cold War anti-communist Gladio operations, now used in Turkey as shorthand for alleged state-sponsored nationalist violence.
Shadowy Turkish nationalist groups are believed to have killed hundreds of activists in the mainly Kurdish southeast over the past three decades in unsolved murders.
Turkish media reports have also suggested the possible involvement of Damascus or Tehran, which have Kurdish minorities and are at odds with NATO member Turkey over issues including the conflict in Syria.
Erdogan said that while investigations needed to be completed before a definitive conclusion could be reached, evidence so far pointed to an internal feud, as the building was secured by a coded lock which could be opened only by insiders.
"Those three people opened it. No doubt they wouldn't open it to people they didn't know," Erdogan told reporters on his plane returning from Senegal on Friday, according to the state-run Anatolian news agency.
He said the killings could also have been intended to sabotage efforts towards peace talks with the PKK.
THREAT TO PEACE TALKS
Cansiz was a prominent figure in the PKK, initially as a fighter and later in charge of the group's civil affairs in Europe. A 1995 photograph shows her standing next to PKK leader Abdullah Ocalan wearing olive battle fatigues and clutching an assault rifle.
French investigators gave no immediate indication of who might be behind the murders. The PKK has seen intermittent internal feuding during an armed campaign in the mountainous Turkish southeast that has killed some 40,000 people since 1984.
Turkish nationalist militants - linked by critics of the military to the security establishment - have in the past also been accused of killing Kurdish activists, who want regional autonomy. But such incidents have been confined to Turkey.
"Kurds don't benefit from this murder. I don't think in-fighting is at all behind this," said Eren Keskin, one of Ocalan's lawyers who first met Cansiz in 1991.
"The Kurdish problem isn't just in Turkey, it involves many states in the Middle East, Europe and the United States. There are many circles that are uncomfortable with the prospect of peace, and there are many who profit from the lack of peace."
Turkey recently announced it had begun talks with Ocalan, jailed since 1999 on the small island of Imrali near Istanbul. PKK hardliners are likely to be sceptical about such talks.
According to media reports, the Turkish state and PKK have agreed the framework for a peace plan, which would involve boosting Kurdish minority rights in exchange for the ultimate disarmament of the militants.
"If the events surrounding this murder aren't revealed, then this process will collapse, sooner or later," Sebahat Tuncel, a BDP lawmaker who knew Cansiz, told Reuters.
"Anyone who knows the Kurdish movement and its history knows it is not possible for the PKK to fracture like this ... In the past, these types of provocations that have derailed peace efforts have come from the state," she said.
Erdogan has introduced reforms allowing Kurdish language broadcasting and other concessions on language; but activists are demanding more freedom in education and administration.
Kurdish politicians are also demanding improved prison conditions for Ocalan with a view to him being released from jail and put under house arrest, but Erdogan played down any changes in Ocalan's situation.
"The conditions at Imrali are better than those in any country in the world and we're talking about special treatment," Erdogan said. Ocalan was able to walk daily in a courtyard with other inmates and would be given a television, he said.