Israeli forces have stopped a Gaza-bound aid boat and detained Greta Thunberg and other activists who were on board, enforcing a longstanding blockade of the Palestinian territory that has been tightened during the war with Hamas.

The British-flagged yacht, Madleen, which is operated by the pro-Palestinian Freedom Flotilla Coalition, hadaimed to deliver a symbolic amount of aid to Gaza later on Monday and raise international awareness of the humanitarian crisis there.

"If you see this video, we have been intercepted and kidnapped in international waters by Israeli occupational forces or forces that support Israel," Thunberg, 22, said in a video released by the FCC, filmed before the vessel was captured.

"I urge all my friends, family and comrades to put pressure on the Swedish government to release me and the others as soon as possible."

The Freedom Flotilla Coalition (FFC), which had organised the voyage to deliver humanitarian aid to the Gaza Strip and protest Israel’s blockade and wartime conduct, said the activists had been “kidnapped by Israeli forces” and released pre-recorded messages from them.

“The ship was unlawfully boarded, its unarmed civilian crew abducted, and its life-saving cargo – including baby formula, food and medical supplies – confiscated,” it said in a statement.

The Israeli Foreign Ministry confirmed that the vessel was under Israeli control. Israel has called Thunberg an "antisemite" and dismissed the aid ship as a stunt.

"The 'selfie yacht' of the 'celebrities' is safely making its way to the shores of Israel. The passengers are expected to return to their home countries," the ministry wrote on X. All passengers were safe and unharmed, it later added:

"They were provided with sandwiches and water. The show is over."

It said the passengers would return to their home countries and the aid would be delivered to Gaza through established channels. It later circulated footage of what appeared to be Israeli military personnel handing out sandwiches and water to the activists, who were wearing orange life vests. 

Thunberg, an environmental campaigner, and Rima Hassan, a French member of the European Parliament, were among 12 activists aboard the Madleen, which set sail from Sicily a week ago. Along the way, it had stopped on Thursday to rescue four migrants who had jumped overboard to avoid being detained by the Libyan coast guard.

The British-flagged yacht Madleen had hoped to reach Gaza later in the day, when the interception occurred, the group said on its Telegram account.

Israeli forces seize Gaza-bound aid boat and detain Greta Thunberg and other activists. - AP

The Israeli Foreign Ministry posted a video online showing the Israeli Navy communicating with the Madleen over a loudspeaker, urging it to change course.

"The maritime zone off the coast of Gaza is closed to naval traffic as part of a legal naval blockade," a soldier said.  "If you wish to deliver humanitarian aid to the Gaza Strip, you are able to do so through the (Israeli) port of Ashdod."

Israeli Defence Minister Israel Katz ordered the military on Sunday to prevent the Madleen from reaching Gaza, calling the mission a propaganda effort in support of Hamas.

Katz said that Israel wouldn't allow anyone to break its naval blockade of the Palestinian territory, which he said was aimed at preventing Hamas from importing arms.

"To the antisemitic Greta and her fellow Hamas propagandists — I will say this clearly: You should turn back, because you will not make it to Gaza," he said in a statement.

Israeli Defence Minister Israel Katz ordered the military on Sunday to prevent the Madleen from reaching Gaza, calling the mission a propaganda effort in support of Hamas.

The yacht was carrying a small shipment of humanitarian aid, including rice and baby formula. The Foreign Ministry said it would be taken to Gaza.

"The tiny amount of aid that was on the yacht and not consumed by the 'celebrities' will be transferred to Gaza through real humanitarian channels," it wrote.

The Swedish foreign ministry said it was in contact with Israeli authorities.

"Should the need for consular support arise, the Embassy and the Ministry of Foreign Affairs will assess how we can best help the Swedish citizen/Greta Thunberg resolve her situation," a Swedish Foreign Ministry spokesperson said in a written statement.

Aid boat activists being given sandwiches and bottled water. – Israel Foreign Ministry via Reuters.

French President Emmanuel Macron's office said he has asked Israeli authorities to release the French nationals on board as soon as possible.

The French and Spanish foreign ministries said they had requested consular protection for their citizens aboard.

After a 2½-month total blockade aimed at pressuring Hamas, Israel started allowing some basic aid into Gaza last month, but humanitarian workers have warned of famine unless the blockade is lifted and Israel ends its military offensive.

An attempt last month by Freedom Flotilla to reach Gaza by sea failed after another of the group's vessels was attacked by two drones while sailing in international waters off Malta. The group blamed Israel for the attack, which damaged the front section of the ship.

• MALTA: Freedom Flotilla attacked by drones

Israel and Egypt have imposed varying degrees of blockade on Gaza since Hamas seized power from rival Palestinian forces in 2007. Critics of the blockade say it amounts to collective punishment of Gaza's roughly 2 million Palestinians.

Israel sealed Gaza off from all aid in the early days of the war ignited by the Hamas-led attack on southern Israel on October 7, 2023, but later relented under US pressure. In early March, shortly before Israel ended a ceasefire with Hamas, the country again blocked all imports, including food, fuel and medicine.

Hamas-led militants killed around 1200 people, mostly civilians, in the October 7 attack and abducted 251 hostages, more than half of whom have since been released in ceasefire agreements or other deals. Hamas is still holding 55 hostages, more than half of them believed to be dead.

Israel's military campaign has killed more than 54,000 Palestinians, according to the Gaza Health Ministry, which has said women and children make up most of the dead. It doesn't say whether those killed are civilians or combatants.