The Tahrir and the Saoirse set sail from the Turkish port of Fethiye on Wednesday with the aim of breaking through the Israeli-imposed sea blockade in order to bring humanitarian aid to Palestinians living in the Gaza strip.
Israel has said it will "take any necessary action" to prevent the ships, carrying 27 activists from nine different countries including the U.S. and Canada, from reaching the port of Gaza city.
"The navy has completed the necessary preparations in order to prevent them from reaching the Gaza Strip," it said in a statement.
The IDF said the purpose of the mission was "to create a provocation against the state of Israel, to break the maritime security blockade on Gaza, and to undermine Israel's security".
"The reality is that this [the Israeli blockage of Gaza] is a policy which a great deal of international law and opinion... deems illegal, immoral and certainly unnecessary. There are many alternatives available to Israel, and imprisoning 1.6m people in a regime of collective punishment is not one of them," said Felice Gelman, a New-York based activist for the Gaza Freedom March.
Israel has maintained an almost total blockade on the Gaza strip since 2007. It imposed the blockade in response to Palestinian rocket attacks against Israel by Gaza-based militant group Hamas.
There is considerable controversy as to whether the on-going blockade is legal or illegal according to international law. United Nations Rapporteur and Princeton University professor Richard Falk has said there exists and "overwhelming consensus" among qualified international law specialist that both the blockade and its enforcement are legal. A UN investigative committee concluded in the Palmer Report, published Sept. 2011, that the blockade is legal.
USTOGAZA says that the ultimate aim of the flotilla is to bring international awareness to the plight of Palestinians living under the Israeli blockade:
"The message to the rest of the world is that people in Gaza have the right to live a normal human life- demonstrates that they have the right to trade, to exports - to receive the things they need to have an economy, which they don't; the supplies they need to take care of their sick, which they don't; and to the resources they need to educated their children, which they don't," Gelman added.
Israeli naval commandos caused a diplomatic crisis with Turkey in May 2009 when they intercepted a flotilla of activists heading to Gaza, leading to the death of nine Turkish nationals.
An attempt to launch a second flotilla from Greece earlier this year was blocked by the Greek government under pressure from Israel and the U.S.
A security expert with links to the Mossad said the activists on the flotilla were both "crazy" and "stupid" to think that Israel would let them get through.
"They get so far and just get sent back again. The Israelis will bring them down. Bang bang bang," he added.
An Israeli air strike killed two Palestinians in northern Gaza on Thursday. The Israeli military confirmed the strike in a statement that said the air force had targeted a "terrorist squad" near Gaza's northern border with Israel.