Organisers of a Gaza-bound flotilla carrying aid as well as a number of Irish and international pro-Palestinian activists said they heard explosions and saw multiple drones target some of their boats.
Italy's Defence Minister Guido Crosetto sent a navy frigate to assist a Gaza-bound flotilla.
"To ensure assistance to the Italian citizens on the 'Flotilla'... I spoke with the Prime Minister and authorised the immediate intervention of the Italian Navy's multi-purpose frigate Fasan, which was sailing north of Crete as part of Operation Safe Sea," Mr Crosetto said in a statement posted by the ministry on X.
"The vessel is already en route to the area for possible rescue operations", he said.
Mr Crosetto expressed "the strongest condemnation" of the "attack" on the flotilla using "drones by currently unidentified perpetrators".
The Global Sumud Flotilla (GSF) departed Barcelona on 31 August, with the aim of breaking Israel's blockade of Gaza.
It currently numbers 51 vessels, most of which are off the Greek island of Crete.
Among its high-profile participants is environmental activist Greta Thunberg.
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There are believed to be 22 Irish people onboard the fleet of boats.
The ministry had already informed Israel that "any operation entrusted to Israeli forces must be conducted in compliance with international law and the principle of absolute caution," the ministry said in a statement.
"Minister Tajani has asked the Italian Embassy in Tel Aviv to gather information and to reiterate its previous request to the Israeli government to guarantee the absolute protection of the personnel on board," it added.
Israel, which blocked two previous attempts by activists to reach Gaza by sea in June and July, said Monday it would not allow the flotilla to reach the embattled Palestinian territory.
Read more: Gaza aid flotilla departs Barcelona again after stormy weather
German human rights activist and flotilla member Yasemin Acar said in a video she posted on Instagram that five vessels had been attacked.
"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.
"We are carrying only humanitarian aid. We have no weapons. We pose no threat to anyone. It is Israel who is killing thousands of people (and) starving a whole population," Ms Acar said.
In an earlier video, Ms Acar said the activists had "sighted 15 to 16 drones", adding that their radios had been jammed as loud music could be heard.
One video posted by the flotilla's official Instagram page showed an explosion it said it recorded from the Spectre boat at "01:43 GMT +3".
In another video posted by the same page, Brazilian activist Thiago Avila said four boats had been "targeted with drones throwing devices" just before another explosion was heard in the background.
The flotilla had already been targeted in two suspected drone attacks in Tunisia, where its boat had been anchored before resuming its voyage towards Gaza.
Israel has come under huge international pressure over its war in Gaza, which has sparked a dire humanitarian crisis in the Palestinian territory.
Last month, a body backed by the United Nations officially declared famine in part of Gaza.
While on 16 September, UN investigators accused Israel of committing "genocide" in the besieged territory, nearly two years after the war erupted following Hamas's 7 October 2023, attack on Israel.
