Pro-Palestinian activists vowed to press ahead and break an Israeli blockade of the Gaza Strip on Thursday by sending in a convoy of relief aid ships despite Israeli warnings it will be stopped.

Eight ships, including four cargo vessels, were heading towards Gaza in defiance of a three-year Israeli closure on the sliver of desert territory, home to 1.5 million Palestinians.

"We are planning on going. This is not going to stop us. The boats are already on their way," said Greta Berlin, a spokeswoman for the Free Gaza Movement.

Organisers said the boats would converge at a meeting point in international waters east of Cyprus, probably late on Friday, and then head across the eastern end of the Mediterranean Sea towards Gaza, Berlin told Reuters.

Israel urged the convoy, led by a passenger ship with 600 people on board chartered by a Turkish human rights group, to turn back, saying its navy was prepared to intercept it.

Naval commandos have held drills in preparation for boarding and searching the convoy. Activists faced arrest and deportation, and their cargo would be confiscated for possible transfer by Israel to Gaza, Israeli military officials said.

"We are working, up until the last minute, to persuade the captains, and other elements on these ships, not to get into this needless, damaging caper," said Yossi Gal, director-general of Israel's Foreign Ministry.

Israel and neighbouring Egypt closed Gaza's borders after Islamist Hamas, which rejects the Jewish state, took over the territory in 2007. Tensions have run especially high since the December 2008-January 2009 war between Hamas and Israel.

Gaza's people, many of them United Nations aid recipients, suffer shortages of water and medicine.

The Free Gaza Movement first started sending aid directly into Gaza in August 2008. They have been intercepted on three occasions, the last time in July 2009 when Israel seized their boat and detained 21 activists.

"The people of Gaza are being held in a concentration camp. When governments refuse to act, it is up to civil society to act," Berlin said.

The convoys would be taking in 10,000 tonnes of supplies, including cement -- a material Israel bans, citing fears Hamas could use it to construct bunkers -- as well as water purification kits, pre-fabricated homes and medical equipment.

"Israel refuses to let anything into Gaza to help rebuild the society they blew to smithereens," said Berlin.

In recent weeks Israel has allowed some goods it used to ban, such as clothes, shoes, wood and aluminium, to enter the strip through land border crossings. It continues to allow a steady flow of humanitarian aid into the coastal territory.

"There is no humanitarian crisis in Gaza," Gal told Israel Radio. "This is a flotilla whose entire purpose is to sail deep into the murky waters of propaganda."