Greta Thunberg's Gaza flotilla was not hit by a drone, Tunisian authorities have insisted, saying activists' claims 'have no basis in truth' after the group alleged their boats had come under attack.

A dramatic video posted by fellow activist Yasemin Acar on Instagram showed the aftermath of a fire, as she declared: 'The Family boat has been officially attacked – a drone came right above it, released a bomb and it exploded and the boat was on fire. 

She added: 'Everyone on that boat is ok. The fire is out.' 

However, Tunisia's interior ministry said that reports of a drone striking a boat at its Sidi Bou Said port 'have no basis in truth', and that a fire broke out on the vessel itself. 

Claims of a drone are 'completely unfounded', the national guard said in a statement on its official Facebook page, indicating that the fire may have been caused by a cigarette. 

Tunisian national guard spokesman Houcem Eddine Jebabli said, 'no drones have been detected'. He told AFP: 'According to preliminary findings, a fire broke out in the life jackets on board.' 

The United Nations' Special Rapporteur on the occupied Palestinian territories, Francesca Albanese, who was at the port, told Reuters: 'We do not know who carried out the attack, but we would not be surprised if it was Israel. If confirmed, it is an attack against Tunisian sovereignty.' 

Thunberg is part of a controversial convoy of around 350 activists and aid workers, sailing on what they describe as a 'peaceful mission' to deliver supplies to Palestinians in Gaza.

But reports emerged overnight that one of the vessels – the so-called Family Boat, flying under a Portuguese flag and carrying members of the flotilla's Steering Committee, including Greta – was attacked 'by a drone and caught fire' off the coast of North Africa.

She added that the ship, which was sailing in Tunisian waters, carried many of the same people who were detained with Greta during an earlier attempt to reach Gaza in June.

'They have bombed a boat with civilians on it in Tunisian territory. This is an attack against Gaza because they don't want us there. So we need you to mobilise. Let's not stay quiet,' she said.

The boat reportedly only incurred minor damage.  

The Global Sumud Flotilla (GSF), the group behind the mission, later confirmed the alleged attack, saying in a statement: 'The Global Sumud Flotilla confirms that one of the main boats, known as the 'Family Boat' – that was carrying GSF Steering Committee members – was struck by what is suspected to be a drone. 

'The boat was under the Portuguese flag and all passengers and crew are safe. An investigation is currently underway and when more information is available it will be released immediately.

'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.'

While no nation had officially claimed responsibility for the incident, Israel had previously described the flotilla's volunteers as 'terrorists', warning that they would be subject to arrest if they attempted to breach the naval blockade.

Speaking from Tunisia on Sunday, where she was welcomed by cheering crowds in the port of Sidi Bou Said as their boat docked there, she declared: '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.'

It comes after a number of drones were spotted flying over the Global Sumud Flotilla a week ago. 

Speaking in another Instagram video, Yasemin said: 'We have detected the first drones...From what we can tell and the information we are getting, every boat right now in the flotilla has a drone above them.'

Her video address came as the flotilla was around 90 nautical miles from the Spanish island of Menorca.

'We are watching them very closely trying to figure out which drones they are and what they are here for', she added.

It comes after Greta was deported by Israel back in June when the ship she was traveling on with 11 other people, the Madleen, was stopped by the Israeli military.

At the time, Israel was accused of using drones to spray an irritant substance on Greta's flotilla before boarding and detaining her and other activists.

Images showed the deck splattered with an unidentified white liquid. Activist Yasmin Acar, among the 12 on board, said it had been deployed by Israel and was affecting her eyes.

Despite her deportation in June, the activist has shown no signs of backing down as she departed from Barcelona on the Global Sumud flotilla on Sunday to deliver humanitarian aid to the Palestinian enclave.

The maritime convoy of about 20 boats and delegations from 44 countries is claimed to be the largest attempt to date to break the Israeli blockade of the Gaza Strip by sea, which has now lasted 18 years.

They will be joined by more ships from ports in Italy and Tunisia in the coming days, on the route from the western end of the Mediterranean to the Gaza Strip.

This comes as Israel has stepped up its offensive on Gaza city, limiting the deliveries of food and basic supplies in the north of the Palestinian territory.

Food experts warned last month that the city was in famine and that half a million people across the strip were facing catastrophic levels of hunger.

The Global Sumud Flotilla is carrying food, water and medicine. Activists on board demanded safe passage to deliver the much-needed aid and the opening of a humanitarian sea corridor, according to a statement.