Israel has warned its military will "take whatever measures are necessary" to prevent a boat carrying pro-Palestinian activists and humanitarian aid reaching Gaza.
The Madleen yacht is 160 nautical miles from the Gaza Strip and is attempting to challenge Israel's naval blockade of the territory.
Defence Minister Israel Katz said the 12-strong crew, which includes climate protester Greta Thunberg, should turn back and that Israel will act against any attempt to breach the blockade.
The Freedom Flotilla Coalition (FFC) said the vessel, which departed Sicily on Friday, was carrying humanitarian aid and was "prepared for the possibility of an Israeli attack".
Katz said the blockade's purpose was to "prevent the transfer of weapons to Hamas" and was essential to Israel's security as it seeks to destroy the Palestinian armed group Hamas.
