The Israeli navy has established radio contact with a Gaza-bound aid ship, informing the crew that Gaza is under blockade and that the ship should change course.
Israel has vowed not to allow the 1,200-ton Rachel Corrie to reach Gaza despite international condemnation over its deadly takeover of another aid ship earlier this week.
However, organizers have said they will not voluntarily head to an Israeli port instead.
The aid ship was just a few dozen miles (kilometers) from Gaza's shore when it was contacted by the Israeli navy.
The military released excerpts from a transcript of what navy officers told the crew of the Rachel Corrie.
The navy said the cargo could be taken to Gaza by land crossings.