A boat carrying Swedish activist Greta Thunberg and other members of a civilian flotilla bound for Gaza was struck and set on fire by what organisers said was a suspected drone, witnesses and the convoy said Wednesday, though all passengers were reported safe.

Yasemin Acar, one of the activists on the vessel known as the "Family Boat," posted a video on Instagram saying "a drone came right above it, released a bomb and it exploded and the boat was on fire." She said the blaze has since been extinguished and that those aboard were uninjured.

The Global Sumud Flotilla (GSF) confirmed in a statement on its Instagram page that the Family Boat, which it said was sailing under a Portuguese flag and carried steering committee members, had been "struck by what is suspected to be a drone." The group said an investigation was under way and that "acts of aggression aimed at intimidating and derailing our mission will not deter us. Our peaceful mission to break the siege on Gaza and stand in solidarity with its people continues with determination and resolve."

The convoy, which organisers say comprises about 20 boats and delegations from 44 countries, is attempting what activists call the largest maritime effort yet to breach Israel's blockade of the Gaza Strip. The flotilla left Barcelona earlier this week and has been joined by ships from ports in Italy and Tunisia. Pictures from the Tunisian port of Sidi Bou Said showed crowds gathered as Thunberg and other activists docked there on the route.

Acar said drones had been observed over the flotilla previously and that as of her message the vessels were roughly 90 nautical miles from the Spanish island of Menorca. The convoy briefly attempted to leave on Sunday but returned to port because of stormy weather.

Thunberg was deported by Israeli authorities in June after an earlier attempt to reach Gaza aboard a different vessel, the Madleen, was intercepted by the Israeli military. Activists then accused the Israeli forces of deploying a drone to spray an irritant on the ship; images circulated after that incident showed the deck splattered with an unidentified white liquid. Israel has previously indicated it may seek to designate some flotilla volunteers as terrorists, statements that organisers have cited as part of the security risks facing the mission.

The flotilla says it is carrying food, water and medicine and has demanded safe passage and the opening of a humanitarian sea corridor to Gaza. Humanitarian agencies have warned that parts of Gaza are facing catastrophic shortages of food and basic supplies amid an intensified Israeli offensive.

There was no immediate comment from Israeli authorities on Wednesday's reported drone strike. The GSF said it would release more information as it became available and reiterated that its mission would continue.