Tunisia on Tuesday denied accusations that a boat in a Gaza-bound flotilla was hit by a drone off its coast, saying the explosion came from inside the vessel.

The Global Sumud Flotilla, an independent aid group, said Monday that one of its main boats heading to the Gaza Strip had been struck by a drone in Tunisian waters.

The explosion caused fire damage to the Portuguese-flagged boat’s main deck and below-deck storage, but all six passengers and crew are safe, GSF said in a statement.

“Acts of aggression aimed at intimidating and derailing our mission will not deter us,” the group said. “Our peaceful mission to break the siege on Gaza and stand in solidarity with its people continues with determination and resolution.”

An investigation is currently underway, according to GSF. With around 20 boats and participants from 44 countries including activist Greta Thunberg, the group is the largest civilian maritime attempt to break the Israeli blockade of the Palestinian territory.

Tunisian officials rejected the allegation, saying the explosion occurred inside the vessel, Reuters reported. A National Guard spokesperson told local media that claims of a drone attack on the flotilla “have no basis in truth.”

Miguel Duarte, a crew member, said the suspected drone stopped close to the flotilla, moved slowly to the front, and then dropped “what was obviously a bomb” on the forward part of the deck.

“Let me be clear 100%. It was a drone dropping a bomb on the forward deck of our ship,” Duarte said when asked whether the team was “absolutely certain that this was a drone attack,” a video he posted on Instagram shows.

“I’m OK, but we could have been killed, right?” Duarte said, citing the deaths during the Israel-Hamas war of humanitarian and medical workers.

“We know the dangers that we’re facing,” he said.

After the explosion, crew members repeatedly yelled for help and sounded the fire alarm, according to a CCTV video shared by Francesca Albanese, the U.N. special rapporteur for the West Bank and Gaza.

“If it’s confirmed that this is a drone attack, it will be an assault and aggression against Tunisia and against Tunisian sovereignty,” Albanese said after the incident. “And again, we cannot keep on tolerating this and normalizing the illegal.”

Demonstrators gathered at the port of Tunis around 2 a.m. local time (10 p.m. Monday ET), with some waving Palestinian and Turkish flags and chanting “Free Palestine,” according to a video reposted by GSF.

Starvation deaths in Gaza surged after Israel’s blockade in March, and aid was resumed in May under a system backed by the U.S. and Israel. Since then, nearly 1,400 people have died and 4,000 more have been injured while seeking food, with at least 859 killed near aid sites, the United Nations says.