The reports come amid soaring tensions in Turkey following Monday's suicide bombing in the town of Suruc on the border with Syria that killed 32 and that the authorities blamed on IS.
The man, named as Mursel Gul, was assassinated late on Tuesday in Istanbul, Turkish media including the Hurriyet daily and state Anatolia news agency reported.
The Patriotic Revolutionary Youth Movement (YDG-H), seen as the youth wing of the outlawed Kurdistan Workers Party (PKK), claimed responsibility for the killing in a statement, reports said.
The group said it had been tracking his movements for three months and alleged Gul had himself been planning attacks in Istanbul.
"We will continue our operations against the IS gang and have identified many of them and will execute and punish them," it added.
"The Suruc murderers will be brought to account," it said.
Anatolia said the the killers, posing as cleaning salesmen, had fired four bullets into Gul's body.
The PKK's military wing had Wednesday claimed the murder of two Turkish police as revenge for the Suruc bombing, raising fears the fighting raging in Syria between Kurds and IS is spilling over onto Turkish territory.
Many Kurds in Turkey are deeply unhappy with the lack of support given by the Turkish government to the fight against IS in Syria and accuse Ankara of collaborating with the group, claims it denies.
Meanwhile, Turkish media reports said an Islamist named Ethem Turkben had been murdered by masked men at his home in the southern city of Adana in a killing also suspected to have been carried out by PKK-allied militants.
It declared a truce in 2013 after the government opened secret peace negotiations with its chief Abdullah Ocalan but the current violence has put this under threat.
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