The French army broadcast anti-ISIS messages to the Libyan people from a frigate in the Mediterranean in June, an officer said on Monday.
The maritime magazine Le Marin said the operation lasted around two days and involved broadcasting messages in Arabic interspersed with poetry and music on an FM bandwidth.
"The aim was to undermine the credibility of the enemy," the officer from a unit which specialises in such operations said, speaking under cover of anonymity.
The army chief of staff confirmed the operation without giving any further details.
The ship made the broadcasts from outside Libyan territorial waters.
France backs the UN-backed Government of National Accord (GNA), which is based in Tripoli. Forces loyal to the GNA are fighting Islamic State militants in their Libyan stronghold of Sirte, 450 kilometres (280 miles) to the east.
The capture of Sirte by ISIS last year sparked fears the militants would use it as a springboard for attacks on Europe, but forces backed by the GNA, supported by US airstrikes, have recaptured nearly all of the city.
France was forced to admit that it has soldiers on the ground in Libya after three French troops were killed there during an intelligence-gathering mission in July.