French prosecutors said on Friday that they had opened a preliminary investigation into the treatment of French activists who have accused the Israeli authorities of abuse after the pro-Palestinian flotilla they were on was intercepted en route to Gaza.

The prosecutors, part of France’s antiterrorism office, said in a statement that they would investigate the activists’ allegations of torture and war crimes. The flotilla’s detention drew increased international attention after an Israeli government minister last month posted a video of himself taunting detained activists.

The Israeli authorities have denied mistreating the activists, who were released and deported to Turkey.

The French investigation comes as Israel faces increasing diplomatic isolation, especially among longtime European allies, over the course of the Israeli military campaign in Gaza that followed the Hamas-led attack on Israel in 2023.

Pro-Palestinian activists have repeatedly tried to challenge Israel’s naval blockade of the Gaza Strip and deliver aid there to alleviate the enclave’s humanitarian crisis, and ramped up those efforts after Israel tightened restrictions during the war.

Last month, a group of activists, including the French ones whose accusations are to be investigated, made another attempt and were intercepted in the Mediterranean. After one ship with that flotilla was halted, Israel’s far-right national security minister, Itamar Ben-Gvir, posted a video of himself taunting activists while they were handcuffed and pinned down to the deck of a ship, drawing outrage both abroad and at home.

In the minute-long video, police officers can be seen manhandling at least one of the detainees while the Israeli national anthem is heard playing. It brought swift and rare criticism from Israel’s prime minister, Benjamin Netanyahu, who said Mr. Ben-Gvir’s conduct was “not in line with Israel’s values and norms.” He said he had ordered the authorities to “deport the provocateurs as soon as possible.”

More than 400 people were detained, including Italian, Spanish, Australian and French nationals, and those countries condemned Mr. Ben-Gvir’s actions. Legal experts have said the treatment of the international activists shown in Mr. Ben-Gvir’s video likely violated international law.

After their release, some of the activists said they had been abused in Israeli detention. A statement released by the Global Sumud Flotilla, which helped organize the action, said some of the activists had been kicked and beaten, while others had been sexually assaulted.

The Israeli military denied “allegations of abuse” by Israeli soldiers “during the operations to protect the legal naval security blockade.” Israel’s prison service, whose guards oversee detainees, said the accusations were “false and entirely without factual basis.”

The New York Times is looking into the activists’ accounts, but has not independently confirmed them.

After the video was posted, France barred Mr. Ben-Gvir from the country. Foreign Minister Jean-Noël Barrot said that while he disapproved of the activists’ approach, “we cannot tolerate that French nationals can be threatened, intimidated or brutalized in this way — all the more so by a public official.”

He called for the European Union to also impose sanctions on Mr. Ben-Gvir, and on Friday Ireland’s justice minister introduced travel bans on him and another far-right Israeli minister, Bezalel Smotrich. Last year, Canada, Britain and a few other countries imposed travel and financial restrictions on Mr. Ben-Gvir and Mr. Smotrich.

Ségolène Le Stradic contributed reporting.