Neither the exact number of fighters nor their location has been confirmed, the official said.
It is unclear where Russia's mercenaries all came from. Some are from Libya and Syria while others are part of the Wagner Group.
The report did not say how many bodies the graves might contain and mission member Chaloka Beyani said a newly appointed forensic expert would try to investigate further.
Russia is reportedly paying foreign mercenaries a monthly salary of $300-600 to fight in Ukraine, and had approved recruitment of 16,000 Middle East volunteers.
Khalifa Haftar's landed briefly in Cyprus before heading to Israel, where it remained for two hours before returning to Cyprus.
The Mossad chief met with the Libyan PM to discuss normalization with Israel, which may be difficult considering the ongoing chaos in Libya.
It is well within the bounds of possibility that by now the Gaddafi regime could again have been in power in Libya, with a son of the former ruler its newly elected president.
More than a dozen migrants drowned trying to cross the Mediterranean to find better opportunities in Europe.
Libya has become synonymous with the kind of chaos that some countries sunk into after the 2011 Arab Spring.
Leila ben Khalifa, an activist who heads the National Movement party, became the first woman to ever run for the Libyan presidency.