The Swedish-owned sailing boat left Naples, Italy, on Oct. 7 with about 20 people from eight countries on board, carrying items including cement, basketballs, musical instruments and theater lighting equipment.
Israel imposed a blockade on Gaza after the militant Islamic Hamas seized control of it in 2007 from its secular rival Fatah. But amid heavy international pressure, it eased the closure in 2010 after a naval raid killed nine Turkish activists on board a Gaza-bound flotilla.
While many consumer goods now flow into Gaza through an Israeli-controlled crossing, some restrictions, including limits on exports, imports of key construction goods and the naval blockade, remain in place. Palestinians and their backers consider the blockade illegal collective punishment against the people of Gaza.
A September 2011 U.N. report upheld the legality of the naval blockade, calling it a "legitimate security measure," although it criticized land restrictions. Other legal experts at the United Nations, along with the Red Cross, have said the blockade is illegal.
Israel says the closure is needed to keep Hamas from obtaining weapons. Gaza militants smuggle weapons into the area through tunnels under the border with Egypt.
The Israeli military has not said whether it plans to stop the boat, only that it will "continue to ensure that the maritime restrictions near the Gaza Strip, instituted to prevent weapons transfers, are maintained."
On the Estelle's website, pro-Palestinian activists cite Israeli media as reporting that the Israeli Foreign Ministry has been in touch with the governments of several countries that have passengers on board the Estelle. "It makes the point that no ship is going to be allowed to approach Gaza," the site said.