Activists on a flotilla attempting to break Israel’s blockade of Gaza said on Wednesday that several of their boats came under drone attack overnight while sailing south of Greece.

The Global Sumud Flotilla reported “unidentified drones and communications jamming” with at least 13 explosions around its vessels. It said drones or aircraft dropped “unidentified objects” on at least 10 boats, damaging sails and equipment but causing no casualties. Activists also posted video of an explosion near one vessel.

Italy condemned the attack and sent a navy frigate toward the area for potential rescue operations. “Demonstrations and protests must be protected when conducted peacefully and in line with international law,” said Italian Defence Minister Guido Crosetto. The European Union also warned against the use of force, while the UN Human Rights Office called for an independent investigation.

Swedish climate activist Greta Thunberg, on board one of the boats, described the strikes as “scare tactics,” saying: “Drones are something Palestinians experience 24-7. That’s what we must focus on.” Other activists reported hearing interference on their radios, including music being played over their communications channel.

The flotilla, involving about 50 vessels from dozens of countries, is carrying food and medicine in a symbolic mission to Gaza. Organisers rejected Israeli accusations of links to Hamas. Israel insists the aid be unloaded at Ashkelon for transfer into Gaza, saying it will not allow breaches of its naval blockade.

The convoy, the largest of its kind, comes amid Gaza’s worsening humanitarian crisis. The 18-year blockade, tightened during the nearly two-year war, has left hundreds of thousands facing famine, according to UN agencies.