Israel's security cabinet has voted to prevent an international flotilla from reaching Gaza, by any means possible.
The group of boats is due to leave Greece in a few days.
Ehud Barak, the Israeli defence minister, says any resulting confrontation will be the responsibility of the organisers.
The Freedom Flotilla II is part of the "Free Gaza movement" - a coalition of human rights groups which sends ships to break Israel's strict naval blockade of the Gaza Strip.
In 2008 it successfully entered Gaza five times.
However, the movement has been intercepted on the past four voyages - including the lethal attack by Israeli forces on the Freedom Flotilla last year on May 31. Nine people, mostly Turkish activists, were killed and many more injured.
Al Jazeera's Barnaby Philips reports from the Greek capital, Athens.