Several vessels belonging to the Global Sumud Flotilla (GSF) carrying humanitarian aid to Gaza were targeted by Israeli drones in international waters south of Crete on Tuesday, organizers said.

“Our boats were repeatedly attacked by military drones. They struck us with unknown but irritating substances, with sound bombs, and even with drones that deliberately damaged the masts of several vessels,” said Maria Elena Delia, the GSF’s Italian spokesperson, in an Instagram post.

“These attacks targeted boats flying Italian, British, and Polish flags. That’s the same as attacking Italy, the U.K., and Poland — essentially a declaration of war against those countries,” added Delia, who was aboard the ship Morgana.

Italian Foreign Minister Antonio Tajani instructed his embassy in Tel Aviv to reiterate his request that Israel “guarantee the absolute protection of the personnel embarked on the Gaza Flotilla.”

The GSF in a press release said that at least 13 explosions were heard. No casualties were reported among the more than 500 people on board the various vessels.

The Global Sumud Flotilla is the fourth and the largest maritime challenge to Israel’s Gaza blockade this year, with 20 ships and over 500 crew, including Swedish activist Greta Thunberg and Irish actor Liam Cunningham, departing from Barcelona. The crew said a previous attack on the flotilla took place on Aug. 9 in Tunisian waters.

Israeli authorities, who have been waging war in Gaza for almost two years now in retaliation for an attack by Hamas militants, have dismissed attempts to ferry aid to the coastal enclave as publicity stunts.