Activists say Israeli troops have boarded a yacht trying to bring humanitarian aid to the Gaza Strip.
“Connection has been lost” on the Madleen, the Freedom Flotilla Coalition campaign group said on the Telegram app.
It posted a photo showing people in life jackets sitting with their hands up. The report could not be independently verified.
Climate activist Greta Thunberg is among those aboard the vessel, which is believed to be off the Egyptian coast.
Israeli Defence Minister Israel Katz earlier ordered the military to prevent the ship from breaking the blockade of Gaza. Israel says the blockade is necessary to prevent weapons from reaching Hamas.