Irish activists on Thursday accused Israel of sabotaging a ship due to join an aid flotilla to challenge an Israeli blockade of the Gaza Strip, forcing the vessel to withdraw.

The Irish Ship to Gaza (ISG) campaign said the ship's propeller shaft had been cut or had a piece gouged out of it, which could have caused it to break and pierce the hull of the vessel once it set sail.

The group said the damage was identical to that suffered by a Swedish-owned ship that had also been due to join the flotilla.

The Irish ship, the Saoirse, had been "sabotaged in a dangerous manner" in the Turkish port of Gocek where it has berthed for the past few weeks, ISG said.

The Irish government said it would take a "very serious view" if sabotage was proved, but said it was up to the Turkish authorities to carry out an investigation.

The Irish group insisted Israel was to blame. "Israel has questions to answer and must be viewed as the chief suspect in this professional and very calculating act of sabotage," it said in a statement.

But Israel dismissed the allegations as "paranoid", saying the flotilla organisers did not have "a shred of evidence" to support their claims.

The Irish group said it would take so long to repair the damage that the Saoirse would not be able to join what is known as the Freedom Flotilla 2. The Irish activists had been re-deployed to other ships.

Israel has repeatedly said it is determined to stop the flotilla.

Israeli troops ended a similar attempt by a multi-national flotilla to reach Gaza in May last year when they stormed the lead vessel, killing nine Turkish activists and sparking a diplomatic storm.

Foreign Minister Avigdor Lieberman on Thursday urged the organisers not to create "new friction" in the region.

Organisers had said Wednesday the flotilla would comprise at least 11 ships, although that figure included the Irish boat.

The Irish group said concerns about the Saoirse first arose on Monday following a short trip near the Gocek marina. Divers and the boat's skippers carried out an inspection on Tuesday.

Photographs on the group's website appeared to show cuts in the propeller shaft.

The vessel's skipper Shane Dillon told AFP the damage could have had disastrous consequences.

"Most definitely if we had sailed and put the engine on the full load, the propeller shaft most likely would have broken and punctured all of the boat and that would have caused it to sink fairly rapidly," he said.

"The pumps wouldn't have been able to take this amount of water."

The Irish activists said the damage was similar to that reported on Monday by the Juliano, another ship that had intended to join the flotilla. It is docked in the Greek port of Piraeus.

"We had a local engineer looking at (the Saoirse), along with myself and many other people and it is identical to the damage that occurred to the Swedish vessel in Greece," Dillon added.

Ireland's deputy prime minister and foreign minister Eamon Gilmore expressed concern about the reports.

"I do have concerns about it and that is something I think that is going to have to be investigated initially by the Turkish authorities," he told reporters in Dublin.

"I will take a very serious view of it if it turns out that there was sabotage of that vessel."

Israeli foreign ministry spokesman Yigal Palmor told AFP: "These are paranoid and ridiculous accusations.

"The organisers have not even a shred of evidence to back up their accusations, but lack of proof has never deterred them from making wide-ranging and vociferous accusations against Israel," he said.

Organisers insisted the flotilla would still set sail despite the alleged sabotage and bureaucratic hitches, but not before this weekend at the earliest.