Video, verified by Al Jazeera, showed an incendiary device falling on the Family Boat on Monday and causing an explosion, but the Tunisian National Guard said no drone was detected near the ship.
The Family Boat is among dozens preparing to travel to Gaza, to break the Israeli blockade on the Palestinian enclave, where a famine has been declared.
A number of prominent politicians and activists are participating in the flotilla, including climate campaigner Greta Thunberg and the former mayor of Barcelona, Ada Colau. Four Italian parliamentarians are also set to join.
The Tunisian delegation to the Global Sumud Flotilla has announced that ships are expected to depart for Gaza from Tunisia’s coastal town of Sidi Bou Said later today.
In a post on social media, the delegation said the departure was scheduled for 4 pm (15:00 GMT), and encouraged people to join the departure events to “show our steadfast support for besieged Gaza”.
Almost all of the boats participating in the flotilla have now arrived in Tunisia and are docking off the coast, including in the port of Sidi Bou Said.
Francesca Albanese, the UN special rapporteur for human rights in occupied Palestine, has shared a post detailing the second attack on the Sumud Flotilla on Tuesday night.
“Video evidence suggests that a drone – with no light so it could not be seen – dropped a device that set the deck of the Alma boat on fire,” Albanese said.
“Expert sources suggest that it was an incendiary grenade wrapped in plastic materials dipped in fuel, which could have been set on fire before landing on the ship,” she added.
In May, The Conscience, a ship sailing with the Freedom Flotilla Coalition (FFC), was hit by two drones, causing the engine to catch fire and blowing a hole in the hull. It was anchored in international waters near Malta at the time of the attack.
In 2010, Israeli commandos raided the Mavi Marmara flotilla ship in international waters. The assault killed 10 activists and injured dozens, leading to global outrage. The ship, owned and run by a Turkish NGO, was carrying humanitarian aid and more than 600 passengers.
Israeli soldiers have also intercepted flotilla ships as they approached Gaza, including both the Madleen and Handala ships this year, and the Marianne in 2015.