A Libyan ship carrying aid and activists is heading for Gaza in a mission that Israel has described as an "unnecessary provocation".
The Israeli navy is monitoring the vessel's progress and preparing to intervene if it continues on a course to Gaza.
"I say very clearly, no ship will arrive in Gaza. We will not permit our sovereignty to be harmed," the foreign minister, Avigdor Lieberman, said in a radio broadcast.
The ship, Amalthea, sailed from Greece at the weekend. It is carrying up to 15 activists and 2,000 tonnes of food and medicine, according to the organisers, a charity chaired by a son of the Libyan leader, Muammar Gaddafi.
The new attempt to break the naval blockade of Gaza comes six weeks after the Israeli navy's lethal interception of a flotilla, in which nine Turkish activists were killed. The assault prompted a wave of international condemnation and resulted in Israel agreeing to ease its blockade of Gaza.
However, it is strictly maintaining its sea blockade, saying it fears that any relaxation could lead to arms being shipped to militants in Gaza.
The Amalthea's captain was quoted as saying: "I haven't received any instructions to change the ship's original course. I am sailing to Gaza." The crew were expecting to reach Gaza on Wednesday.
Well, you knew someone like Muammar al-Gaddafi would try to horn in on all the publicity that MV Mavi Marmara created. Look for more self-aggrandizing Middle Eastern leaders sending aid ships to Gaza. It is a cheap and easy way to show solidarity with Hamas and the Gazans. The Israeli Navy is going to become very busy intercepting all these ships.
There is an odd irony here as the naval blockade of Gaza is now taking on the form of a very iconic sea bird– The Albatross.