The United Nations has demanded an investigation into Israeli drone attacks against an activist flotilla attempting to deliver aid to people in Gaza.

Activists from the Global Sumud Flotilla said Wednesday at least 13 explosions were heard overnight as some of their boats were attacked by drones while sailing south of Greece.

The flotilla said it was targeted by “unidentified drones and communications jamming.” It added that drones or aircraft dropped “unidentified objects” on at least 10 boats.

UN Human Rights Office spokesperson Thameen Al-Kheetan demanded an “independent, impartial and thorough investigation” into the alleged attacks.

“Attacks and threats against those trying to deliver aid to and support the hundreds of thousands of people in Gaza who are suffering famine and starvation defy belief,” he said.

“Such attacks must stop, and those responsible for violations must be held to account,” he added.

No casualties were reported. Activists posted a video showing what appeared to be an explosion on or near one of the boats.

The flotilla, which is carrying activists including Swedish environmentalist Greta Thunberg, is seeking to end Israel’s blockade of Gaza.

It includes about 50 vessels and is carrying a symbolic amount of humanitarian aid, including food and medicine, for Palestinians in Gaza.

The Israeli Foreign Ministry has accused organizers of being linked to Hamas. Organizers have denied the allegations.

Italian Defense Minister Guido Crosetto denounced the attack and activated a navy frigate for possible rescue operations.

“In a democracy, demonstrations and forms of protest must also be protected when they are carried out in accordance with international law and without resorting to violence,” he said.

Thunberg said the activists would not be deterred by the attacks.

“We were aware of the risks of these kind of attacks and that’s not something that’s going to stop us,” she said.

The flotilla has been targeted in several attacks since it set sail from Spain on September 1.

Israel’s blockade and ongoing offensive has pushed Gaza City into famine, according to the world’s top authority on hunger crises.

More than 300,000 people have fled the city in recent weeks as Israel has ordered the population to move south, but an estimated 700,000 remain.