"Connection has been lost" on the Madleen, the Freedom Flotilla Coalition (FFC) campaign group said on the Telegram app, BBC reported.
It posted a photo showing people in life jackets sitting with their hands up.
Appearing to confirm that the ship had been boarded, Israel's foreign ministry claimed the yacht was "safely making its way to the shores of Israel" and its passengers were "expected to return to their home countries".
Climate activist Greta Thunberg is among those aboard the vessel, which was reported to be off the Egyptian coast.
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