Israel intercepted several boats from a flotilla carrying humanitarian aid on Wednesday night as they tried to break Israel’s longstanding maritime blockade of the Gaza Strip, according to statements from flotilla organizers.

“Israeli naval forces illegally intercepted and boarded the Global Sumud Flotilla’s vessel Sirius along with other boats in international waters,” organizers said in a post on the flotilla’s official Instagram account. The statement said that the boats’ communications, including livestreams, had been cut off and that the state of the passengers and crew was unknown.

“This is an illegal attack on unarmed humanitarians,” the post read.

Adnaan Stumo, an American volunteer on the flotilla, said in a voice message that at least three boats had been intercepted and had military personnel aboard. The other boats were still moving forward slowly, he said.

Israel’s foreign ministry posted a video of a woman in uniform talking on a telephone, identifying herself as “the Israeli Navy.” She calls on the flotilla to stop, saying it is “approaching a blockaded zone,” and says its organizers can send aid to Gaza “through the established channels,” urging the flotilla to head to the port of Ashdod, in Israel, instead of proceeding to Gaza.

The ministry says in the post that the flotilla organizers “are not interested in aid, but in provocation.”

The encounter had been long anticipated by the activists on the flotilla, including the climate campaigner Greta Thunberg, who have livestreamed their trip across the Mediterranean Sea on social media. Videos have showed them training for nonviolent responses to potential encounters with Israeli commandos.

The boats are part of the Global Sumud Flotilla, run by activists who are protesting Israel’s war in Gaza by trying to deliver food and other goods to the enclave, which has been under an Israeli blockade since shortly after Hamas seized power there in 2007. Parts of Gaza have been suffering from famine in recent months, according to a United Nations-backed panel of food experts, whose findings Israel has rejected.

The flotilla has drawn wide public attention since it set sail, in part because of the participation of high-profile figures like Ms. Thunberg and lawmakers from Italy, where antiwar protests have snarled traffic and disrupted other activities in recent weeks.

The organizers have been in conflict with Israeli authorities since the mission began. Israel’s government has said that it will do whatever it takes to keep the ships from reaching Gaza and has accused the group of having ties to Hamas, which the organizers have denied.

Global Sumud Flotilla has accused Israel of involvement in a series of drone attacks on the ships as they made their way across the Mediterranean, including in waters near Greece last week and at a port in Tunisia earlier in September.

After those episodes, Italy and Spain sent naval ships to accompany the flotilla for parts of its journey, and Turkey had drones monitor the boats and document potential attacks.

But as the flotilla drew closer to Israel, those countries urged its members to turn back.

On Wednesday, the Greek and Italian foreign ministries issued a joint appeal asking the group to transfer its aid supplies to the Latin Patriarchate of Jerusalem, which would deliver it on their behalf. They said they were concerned that the activists could jeopardize the peace plan that the United States proposed on Monday.

“At this delicate juncture, everyone must refrain from initiatives that could be exploited by those who still reject peace,” they wrote.

Spanish officials also called on the boats not to proceed further into what Israel described as a closed military zone because doing so would “severely jeopardize their own security,” according to Spain’s state broadcaster.

The broadcaster reported that the Spanish Navy would station a rescue vessel nearby in case of emergency, but that the activists had been told that the vessel would not accompany them if they chose to sail further toward Gaza.

Israeli officials had offered to transfer the ships’ cargo to Gaza if they would agree to offload it at an Israeli port or another port in the region. Organizers rejected that offer as disingenuous and said it proved that Israel did not actually consider them to be linked to Hamas.

“After they call us ‘the Hamas flotilla,’ then they invite us to Ashkelon marina,” Mr. Stumo said in a phone interview on Friday.

Mr. Stumo said the volunteers were “normal people” who were disturbed by the civilian impact of the war in Gaza and guided by principles of nonviolence.

Activists’ efforts to reach Gaza by sea have been thwarted before.

In May, a Gaza-bound ship called Conscience halted its mission off the coast of Malta after it was hit by explosions. Israel intercepted the Madleen in June and another vessel, the Handala, in July.

In 2010, an attempt by Israel to stop another flotilla turned deadly after navy commandos boarded one boat, killing at least nine passengers and wounding 30 more.

Israel’s restrictions on goods entering Gaza have become more severe since the war there began after the Hamas-led attack on Oct. 7, 2023. From March to May of this year, Israel blocked all aid from entering Gaza, claiming it was preventing Hamas from looting food intended for civilians.

In August, global food security experts said that parts of Gaza were experiencing famine and that severe hunger and privation were widespread there. Last month, a U.N. commission investigating Israel’s conduct of the war said the country was committing genocide against Palestinians in Gaza.

Israel has rejected both findings and taken issue with their methodology.