The mercenaries drove into Al Sharara oilfield in a convoy of vehicles late on Thursday, the NOC said in a statement on its website, expressing "great concern."
Turkey's top diplomat Mevlut Cavusoglu says mercenaries cannot bring peace and stability to Libya.
Private military contractors offering armies for hire emerged as a powerful stakeholder in the business of war. Governments can outsource compartmentalised jobs to mercenaries or private security contractors, without much international oversight.
The global market for mercenaries and private military contractors is worth more than $100 billion, and the lines that differentiate the two are blurry.
A former US Army sniper and two other ex-American soldiers were found guilty of murder-for-hire and other charges. All three had denied they planned the 2012 execution-style hit in the Philippines.
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