French prosecutors have opened an investigation into war crimes and torture linked to Israel's treatment of French nationals who participated in a Gaza-bound aid flotilla, as scrutiny grows over the mistreatment of activists intercepted during efforts to deliver humanitarian assistance to the blockaded Palestinian enclave.

The probe was opened at the government's request, the national counterterrorism prosecutor's office (PNAT) said, after activists accused Israeli authorities of mistreatment during their detention last month.

Israel detained more than 430 activists from countries around the world after intercepting them in international waters on May 18 as they made the latest in a string of attempts to break the blockade of the Palestinian territory.

Israel's far-right National Security Minister, Itamar Ben Gvir, sparked widespread condemnation after he posted a video mocking the flotilla activists while they were bound.

France banned Ben Gvir from entry over the incident.

Several French activists described what they said was a violent and humiliating ordeal when eight of them returned to France on May 22.

Two of the more than 30 French people who were on board the flotilla were still in hospital in Türkiye, they told reporters.

One returnee described a soldier groping and slapping her in a dark container, and being terrified that she would be raped.

Another recounted detained activists being put in what she called a "stress position," on their knees with their foreheads on the ground for several hours, while the Israeli national anthem played on repeat.

Francesca Albanese, an outspoken U.N. expert on the Palestinian territories, has said the treatment of the flotilla activists "is a luxury compared to what is inflicted on Palestinians in Israeli prisons."