One of the Gaza-bound vessels carrying aid and pro-Palestinian activists including Greta Thunberg was struck by a drone, the flotilla organisers have claimed.

Global Sumud Flotilla (GSF) posted footage on Instagram that appeared to show the Portuguese-flagged vessel being hit by an object, and said all six passengers and crew were safe after the incident outside the Tunisian port of Sidi Bou Said.

Tunisian authorities quoted by Reuters said reports a drone was involved 'had no basis in truth', adding an initial inspection indicated the explosion originated inside the vessel.

The humanitarian aid flotilla set sail from Barcelona last week, and arrived in Tunisia on Sunday.

The GSF said their 'Family boat' was struck in Tunisian waters, and fire had damaged the main deck.

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.

A spokesperson for the Tunisian national guard also told the AFP news agency 'no drone' had been detected and the investigation was continuing.

BBC Verify has authenticated several videos of a claimed 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'.

Regardless of the method of delivery, the impact video casts serious doubt on the Tunisian authorities' version of events.

United Nations Human Rights Council special rapporteur and Tunisian resident Francesca Albanese stated that if an attack could be verified, it would constitute an 'assault and aggression against Tunisia, and Tunisian sovereignty'.

Albanese, who was a featured critic of Israel's military offensive in Gaza, has faced US sanctions, with Israeli Prime Minister Benjamin Netanyahu welcoming them.

Flotilla organisers have clarified that their mission aims to 'break Israel's illegal siege on Gaza', despite facing multiple setbacks in the past, including prior Israeli boarding of activist ships.

Recent reports highlight escalating tensions and a dire humanitarian situation in Gaza, with claims of famine being met with denials from the Israeli government.