Israel said it diverted a Gaza-bound boat on Monday after the activists onboard, including Swedish campaigner Greta Thunberg, said they were intercepted as they sought to provide relief to the blockaded Palestinian territory.
The Madleen departed from Italy on June 1 to bring awareness to food shortages in Gaza, which the United Nations has referred to as the "hungriest place on Earth", with the entire population at risk of famine.
The Israeli government had directed its forces to stop the "celebrities yacht" from reaching Gaza.
AFP lost contact with the activists onboard the Madleen early Monday morning after the organisers said alarms sounded and life jackets were being prepared.
"If you see this video we have been intercepted and kidnapped in international waters," Thunberg said in