An Israeli navy boat intercepted a Libyan aid ship bound for the Gaza Strip, the organizers said.
The ship, organized by the Gaddafi International Charity and Development Foundation, left Greece on Saturday afternoon.
"The Ship's Captain was asked by the gunboat crew to go to El-Arish port instead, stressing that the vessel will not be allowed to go into Gaza at all," the foundation said in a statement on its website.
In response to the Israeli order to head for the Egyptian port of El Arish, "the captain of the Libyan ship and the head of the foundation team on board reaffirmed that the ship's destination is Gaza and no other place," the charity said in a statement.
An Israeli military spokeswoman confirmed that a "process of identification and communication" with the vessel, some 100 miles (160 km) from the Gaza coast, had begun, according to Reuters.
The Moldova-flagged cargo ship Amalthea is carrying 2,000 tons of food and medical supplies with 12-man crew and 15 peace activists.
It is the latest attempt to break the naval blockade on Gaza, and comes six weeks after a deadly Israeli raid ina Turkish-led aid flotilla.
Israeli forces raided a six-ship flotilla carrying humanitarian aid to Gaza on May 31.
Nine people were killed in the raid. The flotilla, which included three cargo ships and three passenger ships, was trying to draw attention to Israel's three-year blockade of the Gaza Strip. The boats were carrying items such as electric-powered wheelchairs, prefabricated homes and water purifiers.
On Amalthea board are a crew of 12 from Haiti, India and Syria, under the command of a Cuban-born captain. Most of the activists on board are from Libya, except for one Nigerian, one Algerian and one Moroccan.
Gazans live under heavy Israel siege for a long time and Egypt still insists on not to opening the only Gaza border crossing in a move condemned by Muslims around the world in protests, leaving Gazans desperate to digging tunnels underground and risking their lives.
Israeli Foreign Minister Avigdor Lieberman said on Sunday that Israel would not allow the Moldovan-flagged Amalthea, renamed Hope, to reach the Gaza Strip, under 3-year heavy blockade of Jewish State.