When they saw what it was about, they refused to continue watching,” said Israel’s defense minister, Israel Katz.

Thunberg was one of 12 passengers on board the boat carrying aid destined for people in Gaza. The activists said they were protesting the war and the humanitarian crisis amid Israeli restrictions on aid.

Shortly after their arrests, the Freedom Flotilla Coalition, the nonprofit that has staged dozens of efforts to reach Gaza to protest Israel, posted prerecorded videos on X showing the crew members saying that they had been “kidnapped.” They called on their home governments to put pressure on Israel to release them.

President Trump took questions before an "Invest America" event at the White House. Mr. Trump commented on reports of activist Greta Thunberg's detention while she was headed to Gaza to distribute aid.