LONDON (Reuters) - A European naval force tasked with patrolling pirate-infested waters off Somalia called on Wednesday for more states to prosecute those intercepted while planning piracy, not only those caught in the act.

The hijacking of ships near the coast of Somalia, where an Islamist insurgency and lawlessness has created a pirate safe haven, has cost the shipping industry millions of dollars, but it is difficult to prosecute those planning an attack.

"We had to let this lot go," said European Union Naval Force operation commander Rear Admiral Peter Hudson, commenting on a photo of alleged pirates. "You can see the ladders, the weapons. So they're not out to go fishing for tuna. The question is which court can I get these rogues into, and there isn't one."

"It would be useful to us if more states were prepared to charge and prosecute on the grounds of conspiracy," he later said, speaking to Reuters after a news briefing in London.

EU NAVFOR's mandate to protect vessels carrying food aid to Somalia, and other ships passing through vital commercial shipping lanes through the Gulf of Aden and near the Somali coast, began in 2008, and has been extended to the end of 2010.

World powers struggle to effectively prosecute captured pirates, even those caught in attacks, either because governments lack jurisdiction or because they fear suspects could seek asylum in the country where they are tried.

Somalia itself lacks the legal infrastructure.

ARREST A DETERRENT?

There are currently eight vessels under Somali pirate control and 157 hostages being held.

EU NAVFOR said successful pirate attacks had dropped sharply through the heavily policed Gulf of Aden, which leads to the strategically important Suez canal into the Mediterranean.

But this has only forced pirates further afield. Small groups of men typically use "mother ships" to sail hundreds of miles out to sea, then launch attacks in small skiffs, armed with assault rifles and rocket-propelled grenade launchers.

Hudson said there had been a surge of pirate activity and "swarms" of pirate groups further down the Somali coast.

He questioned whether EU NAVFOR's activities alone were enough of a deterrent, and urged more international action to get war-ravaged Somalia "back on its feet".

"It's an interesting academic debate as to whether jailing pirates, breaking pirate groups, serves as a deterrent to a 16, 18, 21-year-old youth on the beach of eastern Somalia with very limited life opportunities," Hudson said.

In a sign of greater international cooperation to tackle piracy near Somalia, EU NAVFOR and its partners, NATO and U.S. naval forces based in Bahrain, have held talks with China about it taking a greater role in protecting sea traffic.

Chinese vessels and several other countries currently escort ships vital to their own interests, but the Asian superpower -- which has massively boosted its investment in mineral-rich Africa -- may soon take a role in coordinating shipping protection regardless of the vessel's flag.

"It's being processed through the Beijing authorities and we expected to hear in the not too distant future ... We're optimistic they will join us," Hudson said.