Activists aboard a Gaza-bound flotilla that includes Swedish campaigner Greta Thunberg said early Wednesday that one of their vessels had been struck by what they believe was a drone, in an incident that followed a separate fire aboard another flotilla boat the previous day.

In social media posts and a statement, the Global Sumud Flotilla (GSF) said flames broke out after an object landed on the vessel known to organisers as the "Family Boat." The group said the craft, which it described as carrying members of its steering committee, was sailing under a Portuguese flag and that the fire had been extinguished with no injuries reported. GSF said an investigation was under way and vowed the mission would continue.

Dramatic footage shared online and provided by activists shows a high-pitched sound followed by an object landing on a small boat and igniting a blaze. In the video, a crew member points skyward and dives for cover before the flames are seen on the deck. GSF posted that "another boat has been hit" in the early hours of Wednesday, repeating claims made by activists on board that a similar vessel had been struck by an unmanned aerial vehicle off the Tunisian coast the day before.

Tunisian authorities disputed the accounts. A Tunisian national guard spokesman, Houcem Eddine, said preliminary findings indicated a fire had started in life jackets on board and that no drones had been detected. The Tunisian government has not released detailed forensic findings and said investigations were continuing.

Francesca Albanese, the United Nations special rapporteur on the occupied Palestinian territories, who was present at the Tunisian port where the convoy has been anchored, said she did not know who was responsible but added she "would not be surprised if it was Israel." Albanese said if the allegation were confirmed, the incident would amount to an attack on Tunisian sovereignty.

The flotilla, organised by the Global Sumud Flotilla, arrived in Tunisia over the weekend and has been anchored off the north African country as it seeks to sail to Gaza to deliver aid and challenge Israel's maritime blockade. The convoy has been delayed several times by weather and logistical problems; organisers said they planned to resume the voyage on Wednesday.

Among the high-profile participants is climate activist Greta Thunberg, who addressed pro-Palestinian campaigners in Tunisia on Sunday and said, "We all know why we are here. Just across the water there's a genocide going on, a mass starvation by Israel's murder machine," remarks she and organisers have used to underline the humanitarian aims of the mission. Organisers say the flotilla is carrying food, water and medicine and are seeking safe passage and the opening of a humanitarian sea corridor for Gaza.

The reported strike follows earlier confrontations involving flotillas to Gaza. In June, Thunberg was deported by Israeli authorities after the ship she was travelling on was stopped; activists then accused Israeli forces of using drones to disperse an irritant substance on deck. Israel has previously said it enforces a naval blockade for security reasons and has intercepted vessels attempting to breach it.

There has been no independent confirmation of who carried out the suspected attacks off Tunisia. GSF said it would release more information when available and called the incidents "acts of aggression aimed at intimidating and derailing our mission." Tunisian investigators and international monitors said inquiries were ongoing.