Of the 37 car and suicide bombings recorded this year in Libya, 27 have been attributed to the IS group, Bensouda said in a report to the UN Security Council.
IS fighters are executing Libyans for "perceived activities such as spying, homosexuality and social activism," she said.
Violent deaths are on the rise in Libya, where the United Nations is trying to broker a deal on a unity government that would be able to confront the growing threat from the Islamic State group.
No fewer than 60 people are killed per month in the north African country, which has been in turmoil since the fall of Moamer Kadhafi in 2011.
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IS attacks have hit mostly the southern city of Sirte, but fighting in June drove out the jihadists from Derna, in the east.
Executions and other murders of Libyans "attributed to ISIL and its allied organizations have been consistently more highly numbered than those of other perpetrators," Bensouda said in her report to the council.
In another indication of the worsening crisis, nearly 450,000 people have been forced to flee their homes in the violence over the past year, double the number of displaced from the previous year, she said.
Bensouda said the Libya Dawn militia that controls Tripoli and the Libyan National Army loyal to the internationally recognized government were also committing "large-scale crimes."
The prosecutor told the council she was ready to open up new investigations for possible war crimes, but added that her work was hampered by funding problems.
Libya has had two administrations since August 2014, when the Islamist-backed militia alliance overran Tripoli, forcing the government to take refuge in Tobruk, in the east of the country.