Two vessels that are part of a Gaza-bound flotilla carrying aid and pro-Palestinian activists including Greta Thunberg have been struck in separate suspected drone attacks, flotilla organisers have claimed.

Global Sumud Flotilla (GSF) posted footage on Tuesday appearing to show the first boat being hit by an object outside the Tunisian port of Sidi Bou Said.

Tunisian authorities disputed a drone was involved and said an initial inspection indicated the explosion originated inside the vessel.

GSF then released footage appearing to show a second boat being hit later over Tuesday night, saying it “sustained fire damage on its top deck”. Tunisian authorities are yet to comment on the second claim.

No injuries were reported in either incident, organisers of the humanitarian aid flotilla said.

The vessels set sail from Barcelona last week and arrived in Tunisia on Sunday.

On Tuesday, the GSF said their “Family boat” had been struck in Tunisian waters, and fire had damaged the main deck of the Portuguese-flagged boat.

In a series of videos published to their Instagram, spokespeople for the GSF said an “incendiary device” caused a fire onboard the vessel, which the crew was able to extinguish.

Tunisian authorities quoted by Reuters said reports a drone was involved “had no basis in truth”. A spokesperson for the Tunisian national guard told the AFP news agency “no drone” had been detected and the investigation was continuing.

Over Tuesday night, GSF posted a separate video claiming a second boat had been struck in a suspected drone attack off Tunisia’s coast.

GSF said the British-flagged Alma vessel had been docked in Tunisian waters when it was “attacked by a drone”, organisers said in a statement.

It added: “The fire has since been extinguished, and all passengers and crew are safe.

“An investigation is currently under way and when more information is available it will be released immediately.”

BBC Verify has verified CCTV footage of a strike on the Alma ship just off the coast of Tunisia that was published on GSF’s Instagram account.

Three white storage blocks can be seen at the rear of the Alma – which tally with stock photos of the vessel along with the railings either side.

The Tunisian coastline around Sidi Bou Said can also be seen.

BBC Verify has also authenticated several videos of the claimed first strike on the “Family” vessel off the coast of Tunisia.

David Heathcote, head of intelligence at McKenzie Intelligence Services, told the BBC the nature of the impact angle in footage implies “that the object has been dropped, rather than launched or fired”.

“If so, it could be that a drone was used to manoeuvre over the ship before the package was released on to it,” he said.

“Regardless of the method of delivery, the impact video casts serious doubt on the Tunisian authorities’ version of events,” Mr Heathcote added.

UN Human Rights Council special rapporteur and Tunisian resident Francesca Albanese, who was also featured in videos shared by the GSF, said if an attack could be verified it would be an “assault and aggression against Tunisia, and Tunisian sovereignty”.

Albanese said she was at Sidi Bou Said port and “trying to figure out the facts with local authorities”.

She has been a prominent critic of Israel’s military offensive in Gaza, and has been subject to sanctions imposed by the US in July – a decision welcomed by Israeli Prime Minister Benjamin Netanyahu who called them a “firm measure against the mendacious smear campaign” against Israel.

Flotilla organisers have said the aim of their mission is to “break Israel’s illegal siege on Gaza”, but have faced several setbacks.

In June, Israeli forces boarded a boat carrying humanitarian aid to Gaza and detained the 12 activists onboard, including Swedish activist Thunberg.

Israeli authorities escorted the activists to the port of Ashdod before deporting them from the country.

Israeli authorities have characterised the attempts to sail aid to Gaza as publicity stunts that offered no real humanitarian assistance.

There have also been prior allegations of drone attacks on aid ships bound for Gaza; the Freedom Flotilla alleged that its ship The Conscience was struck by a drone in May off the coast of Malta.

The BBC was sent a recording of the distress call from the flotilla ship, recorded by a crew member on a nearby oil tanker. The captain of the flotilla ship can be heard reporting drone strikes and a fire onboard.

The Maltese government had said everyone aboard the ship was “confirmed safe” and that a fire onboard the ship was “brought under control overnight”.

Last month a UN-backed body confirmed that there was famine in Gaza and the UN’s humanitarian chief said it was the direct result of Israel’s “systematic obstruction” of aid entering Gaza.

The report was labelled an “outright lie” by Israel, which has denied there is starvation in the territory.

In March, it introduced a nearly three-month total blockade on supplies entering the Strip, claiming the aid was being taken by Hamas.

It started allowing a limited amount of aid back into the territory after increasing international pressure.

Israel has since tried to impose its own distribution system through the controversial Gaza Humanitarian Foundation, which has been criticised by aid agencies.

In 2010, Israeli commandos killed 10 people when they boarded Turkish ship Mavi Marmara which was leading an aid flotilla towards Gaza.

The Israeli military launched a campaign in Gaza in response to the Hamas-led attack on 7 October 2023, in which about 1,200 people were killed and 251 others were taken hostage.

At least 64,522 people have been killed in Israeli attacks in Gaza since then, according to the territory’s Hamas-run health ministry.