The Libyan charity's ship was stalled as questions remained over whether it planned to defy an Israeli naval blockade of the Palestinian territory.
The charity, headed by the son of Libyan leader Moammar Gadhafi, said Amalthea left Greece on Saturday, Jul 10, carrying 2,000 tons of food and medical supplies.
Israeli government spokesman Mark Regev had repeated a standing offer, inviting the activists to sail to the Israeli port of Ashdod and unload the supplies there.
The deaths of nine pro-Palestinian activists in the May 31, 2010 raid focused international attention on Israel's blockade of Gaza, imposed after the Islamic militant and anti-Israeli Hamas overran the Palestinian territory in Jun 2007.
The international criticism forced Israel to ease its land blockade of the territory but it has maintained the naval embargo, insisting it is vital to keep weapons out of Hamas' hands.
The Gaza blockade and the challenges to it have caused Israel serious diplomatic damage, putting it on the defensive against demands for inquiries and criticism for its role in Gaza's plight.