JERUSALEM/LONDON, Sept 8 (Reuters) - Anticipating fresh bids by pro-Palestinian activists to sail against its Gaza Strip blockade, and hoping to avoid a repeat of its bloody boarding of a Turkish aid ship in May, Israel has turned to maritime law.

Israeli officials say vessels will be warned in advance that they face costly impoundment and the possible detainment of crews -- a strategy first floated in July when the threat of such action helped turn a Libyan-chartered ship away from Gaza.

"The legal approach proved effective as a deterrent, and we're prepared to see it through as a punishment," said one senior government official, declining to be named.

"If we are to pay a price for defending the blockade, the other side will pay a price for challenging it."

Israel triggered world outcry in May after its forces killed 9 Turks in brawls aboard the Mavi Marmara, a converted cruise liner that tried to lead an aid flotilla to impoverished Gaza, whose 1.5 million Palestinians are ruled by Hamas Islamists.

Israel says its blockade is needed to prevent weapons from reaching Hamas, which refuses to recognise Israel.

Maritime specialists say international law allows a sovereign nation to seize a vessel that runs a declared blockade. That could discourage firms renting out ships to pro-Palestinian campaigners but, arguably, less so in cases like the Mavi Marmara, where the activists buy the ship outright.

"Many ship owners will look at the possibility or likelihood that their vessel will be impounded or fined or seized outright," said J. Peter Pham, a strategic adviser to the U.S. and European governments.

"Those are losses for which they are not insured and for which the non-governmental groups are not going to make good on for them."

THREATS AT SEA

In July a charity belonging to one of Libyan leader Muammar Gaddafi's sons chartered the cargo ship Amalthea for a Gaza aid run. Israeli media aired recordings of the navy warning the captain he would be held responsible for any showdown at sea.

According to Israeli and Libyan officials, a dispute ensued aboard the ship over where to dock. The captain and crew prevailed, taking the Amalthea to El Arish in nearby Egypt.

"Israeli authorities, through the navy, threatened legal action against the ship," said Alex Angelopoulos of Greece's ACA Shipping Corp, which owns the Amalthea.

"The threats were taken into consideration, but the decision (to change course) was taken after long talks with the ship's charterers," he said, adding that were the ship impounded it would mean losses of some 5,000 euros ($6,350) a day.

Another Israeli official said permanent confiscation of ships was "out of the question".

"It would simply be untenable both legally and practically. But we can exert pressure by virtue of the fact that a ship owner stands to lose a lot of money for each day of temporary impoundment in Israel, not to mention the long-term cost of the vessel going rusty and neglected," the official said.

The Israelis say they are bracing for possible new Gaza flotillas by groups from Lebanon, Europe and the United States.

Activists might be willing to lose their ships if they thought their actions would bring them global publicity or dent Israeli prestige -- as happened with the May flotilla.

But some shipping experts doubt whether impoundment could be a feasible long-term solution for Israel.

"The real question should be: how long can Israel detain the ship? My guess is not long without positive proof that there was something more than humanitarian aid on board," said John Dalby, chief executive of maritime asset recovery firm MRM.

Daniel Reisner, a former commander of the Israeli military's international law department, said he knew of no legal basis to try foreigners specifically for violating the Gaza blockade.

"In general, the more innocent the cargo, the more this diminishes the chances of a prosecution," he said.

Of the seven ships in the Mavi Marmara flotilla, four remain berthed in Israel after the three Turkish-owned vessels were returned unconditionally as a fence-mending gesture to Ankara. Israel's Defence Ministry said unspecified "private entities" had sought to reclaim two of the impounded ships but had yet to furnish proof of ownership. The remaining two ships have gone unclaimed while Israel tries to locate their owners.

"It's a double-edged sword: if you have no hope of recovering the actual impound fees then you are stuck with a boat that you neither want nor will be able to get rid of easily," said Pham.