An Irish ship has dropped out of the pro-Palestinian flotilla planning to break Israel's blockade of Gaza, citing sabotage, and a US ship declared it may set sail from a Greek port for Gaza without permission, activists said.

Up to 400 international activists had been due to sail this week to Gaza aboard 10 ships leaving from Greece to protest against the naval blockade that Israel imposed on the Palestinian territory after Hamas militants overran it in 2007.

An Israeli raid on a similar flotilla last year killed nine activists on a Turkish ship and each side blamed the other for the violence.

The Irish ship MV Saoirse yesterday had to abandon plans to set sail because of what it called Israeli sabotage.

Activist Huwaida Arraf told Israel's Army Radio that the ship's engine was damaged while in port and could have lead to deaths on board.

"When the engine was started, it completely bent," Ms Arraf said. "While out at sea, if this would have happened, if it would have bent in this way, the boat would have started taking on water and it could have led to fatalities."

The alleged sabotage occurred at the Turkish coastal town of Gocek where the MV Saoirse has been berthed for the past few weeks, organisers said.

Earlier this week, activists said Israeli agents damaged the propeller of a Swedish ship in the Greek port.

Israel has refused requests for comment on the allegations.

US State Department spokesman Mark Toner - asked to comment on the alleged sabotage - said there was no proof it had taken place due to a lack of "independent confirmation".

Mr Toner, speaking to reporters in Washington, said State Department opinion is "the flotillas are a bad idea," and there're other ways to get this kind of assistance to Gaza.

"Our bottom line is, we don't want to see anyone put at risk. And that would include any action to sabotage these boats but also the boats themselves - the flotillas themselves - will put these individuals at risk."

Yesterday, American activist Ann Wright said Greek authorities were bowing to Israeli pressure and deliberately stalling the departure of 53 passengers and crew - including 11 journalists - aboard a ship named the Audacity of Hope from a port near Athens.

"We're fed up with it already, we've already waited seven days. We were ready to go a week ago," Ms Wright said.

She wouldn't say how long activists would wait for the all-clear before the 33-metre ferry begins its journey. But she said the Greek coast guard would "probably" stop the boat shortly after departing.

Audacity of Hope passenger Alice Walker, author of the acclaimed novel The Colour Purple, read to journalists letters and cards that American children are sending to Gaza residents via the ship.

Israeli prime minister Benjamin Netanyahu spoke about the flotilla at a graduation ceremony for new pilots in central Israel, praising the action of Greek prime minister George Papandreou.

"Sometimes we are required not only to repel physical attacks by our enemies but also repel the attacks against our right to defend ourselves. In that regard, I want to thank the many world leaders who spoke out and acted lately against the flotilla provocation," he said.

"Israel has the full right to act against attempts to legitimise the smuggling of missiles, rockets and other weapons to the terror enclave of Hamas," he added.

The UN's top human rights official welcomed the Israeli government's decision to drop its threat to issue deportation orders against journalists on the flotilla.

"I'm always taking the view that the blockade of Gaza is illegal and must be lifted, since it results in a disproportionate use of force against civilians," UN High Commissioner for Human Rights Navi Pillay told reporters in Geneva.

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