Activists involved in a flotilla seeking to break the Israeli blockade of Gaza say some of their boats have been attacked by drones – and even bombarded with ABBA music – in an overnight raid.

The Global Sumud Flotilla said it was attacked during Tuesday night by “unidentified drones and communications jamming” as it sailed near Greece.

It said “at least 13 explosions” were heard on and around several flotilla boats, while drones or aircraft dropped “unidentified objects” on at least 10 boats.

“The international waters felt like a battlefield,” human rights activist Yasemin Acar, who is onboard the Alma vessel as part of the flotilla, told NBC News on Wednesday (US time).

“There were bombs after bombs … and no one came to help.”

Activists posted a brief video on the flotilla’s social media account showing what appeared to be an explosion on or near one of the vessels.

Acar posted her own video, in which music from pop group ABBA, whose members, like activist Greta Thunberg, hail from Sweden, can be heard.

“They started jamming our comms by playing ABBA songs, you know, to interfere with our radio,” Acar said.

She said the crew “couldn’t even reach the Coast Guard” because of the apparent interference.

“It was horrific that this can happen in international waters, close to Greece. I mean, in Europe,” she said.

A flotilla spokesperson said no casualties were reported but there was damage to the vessels and “widespread obstruction in communications”.

Greece’s coast guard did not report any distress calls.

The Israeli military did not immediately respond to questions regarding the attack.

Italy condemned the attack and activated a navy frigate to be on hand for possible rescue operations, Italian Defence Minister Guido Crosetto said.

The frigate, which had been sailing north of Crete, was “already heading towards the area for possible rescue operations”, he said.

Italy has informed Israel about the decision.

“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,” Crosetto said.

Italy had warned Israel to respect the rights of Italian activists taking part in the flotilla.

Foreign Minister Antonio Tajani said there were 58 Italians involved in the flotilla, including some MPs.

Thunberg, who is on board one of the flotilla boats, called the strikes a “scare tactic” and implored supporters to stay focused on Gaza rather than on the activists sailing in the Mediterranean Sea.

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

Simone Zambrin, an Italian activist with the flotilla, said drones “have been flying over our heads for days now” and on Wednesday “dropped devices at our boats, damaging both the sails and the hearing of some of our crew members”.

The flotilla, comprised of dozens of boats from several countries, is carrying a symbolic amount of humanitarian aid, including food and medicine, for Palestinians in Gaza.

The Israeli foreign ministry accuses organisers of being linked to Hamas, and says it has proposed that the activists unload their aid in the Israeli port of Ashkelon for it to be transported into Gaza.

“Israel will not allow vessels to enter an active combat zone and will not allow the breach of a lawful naval blockade,” the ministry said on Monday.

The flotilla has reported several attacks since it set sail from Spain on September 1, including two while some of its boats were in Tunisian waters.

-with AAP