Israel's Katz said he instructed the military to "take whatever measures necessary" to stop the ship carrying Thunberg and other activists.

Thunberg, a climate campaigner, is among 12 activists aboard the Madleen, which is operated by the Freedom Flotilla Coalition.

Thunberg said she joined the Madleen crew to "challenge Israel's illegal siege and escalating war crimes" in Gaza and highlight the urgent need for humanitarian aid.