A fire that broke out on a Gaza-bound flotilla carrying Greta Thunberg and other anti-Israel activists was likely caused internally, possibly by a cigarette, Tunisian officials said, disputing the passengers’ claims that they were targeted in a drone strike.

“According to preliminary findings, a fire broke out in the life jackets on board a ship anchored 50 miles from the port of Sidi Bou Said, which had come from Spain,” a Tunisian national guard spokesman, Houcem Eddine Jebabli, told AFP. “The investigation is ongoing and no drone has been detected.”

The Tunisian national guard stated on its official Facebook page that reports of a drone attack were “completely unfounded” and suggested instead that the fire may have been caused by a cigarette butt. 

The incident occurred late Monday evening aboard the main vessel of a fleet organized by the Global Sumud Flotilla to challenge Israel’s maritime blockade of Gaza. The Portuguese-flagged ship, known as the “Family Boat,” was scheduled to depart for Gaza on Wednesday, carrying Swedish climate activist Greta Thunberg and other prominent protesters.

The organization released a statement that evening confirming the vessel sustained damage to the deck and below-deck storage, but that the ship’s six crew members successfully extinguished the fire without injury. The group claimed that the vessel had been “struck by a drone in Tunisian waters.”

“Acts of aggression aimed at intimidating and derailing our mission will not deter us,” the Global Sumud Flotilla wrote. “Our peaceful mission to break the siege on Gaza and stand in solidarity with its people continues with determination and resolve.”

One of the boat’s passengers, Portuguese activist Miguel Duarte, described the experience in video testimony he shared on Instagram. Mr. Duarte said that he was “one hundred percent” sure that “it was a drone dropping a bomb on the forward deck of our ship.” 

Footage of the incident shared by the flotilla’s organizers appears to show a flash of light landing on the boat and igniting a fire. 

Israel has maintained a naval blockade on the Strip since Hamas took control of Gaza in 2007, aiming to prevent the terror group from importing weapons via the sea. Egypt, which borders the Gaza Strip along its southwestern edge, sealed its border crossing at the same time and has largely kept the blockade in place since then.

The voyage marks Ms. Thunberg’s second attempt to reach Gaza in protest of Israel’s military campaign against Hamas. In June, Israel intercepted the Gaza-bound Madleen, which was carrying the Swedish activist and other prominent protesters. The passengers — whom Israeli officials dubbed the “celebrities” of the “selfie yacht” — were towed to Israel and deported from the country.

Prime Minister Benjamin Netanyahu held a meeting last month to plan the government’s response to this latest flotilla, which is expected to include approximately 50 ships.

The Israeli national security minister, Itamar Ben-Gvir, suggested taking a more aggressive approach to the fleet to deter future flotillas. He proposed detaining activists “in terrorist-level conditions” without “television or radio” or “superior-quality meals,” and “involving prolonged custody rather than merely overnight detention.”