Athens, Greece - Organizers of a Gaza-bound flotilla carrying aid and pro-Palestinian activists said late Tuesday they heard explosions and saw multiple drones that targeted some of their boats, currently situated off Greece.

"Multiple drones, unidentified objects dropped, communications jammed, and explosions heard from a number of boats," the Global Sumud Flotilla said in a statement, without adding whether there were any casualties.

"We are witnessing these psychological operations firsthand, right now, but we will not be intimidated," the statement said.

German human rights activist Yasemin Acar said in a video she posted on Instagram that five vessels had been attacked.

"We are carrying only humanitarian aid," she said. "We have no weapons. We pose no threat to anyone. It is Israel who is killing thousands of people [and] starving a whole population."

The flotilla, which set sail from Barcelona earlier this month with the aim of breaking Israel's blockade of Gaza and delivering aid to the territory, had already been targeted in two suspected drone attacks in Tunisia.

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

Israel said Monday it would not allow the flotilla to reach Gaza.

Israel blocked two earlier attempts by activists to reach Gaza by sea in June and July.