Questions emerged Friday about whether the U.S. Navy can continue to hold a group of suspected pirates captured on the high seas and kept for now in floating jail cells ringed with barbed wire.

At issue are nine men, probably Somalis, seized Thursday from a small skiff in the Gulf of Aden. A Navy ship fired warning shots and sailors boarded the skiff and arrested the men after a distress call from an Indian-flagged merchant ship.

Although defense officials would not be specific, several acknowledged there may insufficient evidence to hold the men for trial and that some or all might be returned to Somalia.

If a case can be brought, it would be among the first under a new arrangement with Kenya to take on prosecution of suspected Somali pirates.

Somalia has no effective government or recognized court system, and until now foreign navies have been reluctant to detain suspects because of legal uncertainties over where they would face trial.

"They obviously had some strong suspicions about these individuals and are right now mulling through the evidence they have to determine whether or not they can be prosecuted," Pentagon spokesman Geoff Morrell said Friday. "If there is insufficient evidence to do so, they'll have to make another determination, and that could well be repatriating them."

Morrell said the men would not be allowed to keep weapons including a rocket-propelled grenade launcher seized from their boat.

For now the group is being held aboard the Lewis and Clark, a U.S. Navy dry cargo and ammunition ship equipped with bare-bones holding cells. Pentagon video footage showed thin black pallets and pillows spread on the floor, surrounded by coils of barbed wire.

The nine suspected pirates are the second of two groups seized by the Navy this week off the pirate-infested coast of Somalia. Those seizures were part of a high-profile international crackdown on Somali pirate bands that have grown increasingly bold and efficient in taking over merchant ships.

Defense officials said there are not the same concerns about the strength of evidence against the first group seized by the United States. Those seven suspected pirates were detained Wednesday after they allegedly tried to board a merchant ship flagged in the Marshall Islands.

The Pentagon said there is no deadline by which a new U.S.-led anti-piracy consortium must decide what to do with the men, who in the meantime were being treated "humanely."

Separately, the Russian navy said Friday it detained 10 suspected pirates closing in on an Iranian-flagged fishing trawler. Russian military prosecutors were questioning the men, who were caught on Thursday with rifles, grenade-launchers, illegal narcotics and a large sum of money, the Russians said.

Piracy off Somalia, which has not had a functioning government since 1991, reached record levels last year. Somali pirates, seeking multimillion-dollar ransoms, launched 111 attacks and seized 42 vessels last year, mostly in the Gulf of Aden, with attacks peaking between September and November.

Somali piracy accounted for the bulk of the 49 vessels hijacked and 889 crew members taken hostage around the world in 2008, the highest worldwide figures since the London-based International Maritime Bureau began keeping records in 1991.