Organizers of a Gaza-bound flotilla carrying aid and pro-Palestinian activists said late Tuesday that another of their boats had been struck in a suspected drone attack off Tunisia's coast, the second in 24 hours.

The interior ministry said that it was launching an investigation on the incidents.

It marked the first acknowledgement by Tunisian authorities of the attack, after the national guard had earlier denied the presence of any drones.

"Second night, second drone attack," Melanie Schweizer, one of the flotilla coordinators, told AFP on Tuesday.

Aiming to break Israel's blockade of Gaza, the flotilla was to resume its voyage on Wednesday, but was delayed yet again due to weather conditions.

"We will tell you what the best time is to sail according to the weather," Brazilian activist Thiago Avila said Wednesday.

"Hopefully we will leave tomorrow [Thursday] morning, that is what we expect," he added.

The British-flagged Alma was docked in Tunisian waters on Tuesday, when it was "attacked" and "sustained fire damage on its top deck", the Global Sumud Flotilla said in a statement. It added that no one was hurt.

AFP journalists at the scene saw a boat in the distance surrounded by Tunisian law enforcement vessels with flashing lights.

The incident comes a day after the activists said another of their boats was hit by a similar suspected UAV attack. But Tunisian authorities said "no drones" had been detected.

Francesca Albanese, UN special rapporteur for the occupied Palestinian territories, posted video of the burning Alma and said it indicated a UAV attack. 

"Video evidence suggests a drone -- with no light so it could not be seen -- dropped a device that set the deck of the Alma boat on fire," she wrote on social media.

The flotilla arrived in Tunisia at the weekend and was anchored off the coast of Sidi Bou Said, north of Tunis, when it reported the first incident.

Some members of the flotilla said they saw the drone, adding that the boat's bow caught fire immediately after.

Tunisian authorities had dismissed reports of a drone strike as "completely unfounded", suggesting the fire may have been caused by a cigarette.

Tunisian national guard spokesman Houcem Eddine Jebabli told AFP "no drones have been detected".

Security footage posted by the flotilla organizers showed a burning mass falling from a distance onto the ship.

The activists did not point the blame at Israel for the suspected attacks in their statement but said "they come during intensified Israeli aggression on Palestinians in Gaza, and are an orchestrated attempt to distract and derail our mission".

The Israeli military did not respond to AFP's request for comment on Tuesday.

The United Nations declared famine last month in parts of Gaza, warning that 500,000 people face "catastrophic" conditions.

Sumud means "resilience" in Arabic, and the flotilla describes itself as an independent group not linked to any government or political party.

Among its high-profile participants is environmental activist Greta Thunberg.