WASHINGTON (AFP) - President Barack Obama vowed Monday to work with US allies to "halt the rise of piracy" off Somalia's coast, as the Pentagon warned there was no purely military solution to the seaborne scourge.

As the US public savored the daring rescue of an American captain, in which US Navy snipers killed three of his four captors, Washington grappled with ways to blunt the headline-grabbing threat to international shipping.

"I want to be very clear that we are resolved to halt the rise of piracy in that region," Obama said at an event designed to highlight investments in transportation infrastructure.

His remarks were his first in public on the rescue of Maersk Alabama cargo ship Captain Richard Phillips, held hostage aboard a lifeboat adrift off Somalia's coast in a harrowing five-day standoff.

"His safety has been our principal concern, and I know this came as a welcome relief to his family and his crew," said Obama, who telephoned the skipper's wife on Easter Sunday.

Andrea Phillips thanked Obama and other officials for their help in the rescue, and in a statement read by a friend - Phillips had laryngitis - reminded everybody "that there are many more families going through what the Phillips have endured presently and those families are in the prayers of the Phillips family."

Ten Italians, five Romanians and a Croat seized on an Italian tugboat on Saturday were still in pirate hands Monday, while some 200 other crewmembers are reportedly being held by Somali pirates.

US officials moved to cobble together a fresh strategy for battling piracy, reaching out to other capitals, mulling ways to prosecute captured pirates like the lone survivor among the captain's captors and assessing the prospects for stabilizing Somalia and drying up the flow of piracy recruits.

"There is no purely military solution," US Defense Secretary Robert Gates told a group of 30 students and faculty members at the Marine Corps War College in Quantico, Virginia, in remarks first reported by a military news service.

"I am confident we will be spending a lot of time in the situation room over the next few weeks trying to figure out what in the world to do about this problem," said Gates.

Gates said Phillips's captors were 17-19 years old - "untrained teenagers with heavy weapons" - drawn to piracy by a lack of economic alternatives.

"As long as you've got this incredible number of poor people and the risks are relatively small, there's really no way in my view to control it unless you get something on land that begins to change the equation for these kids."

At the Justice Department, spokesman Dean Boyd said the lone pirate survivor, who had surrendered before the bloody rescue, could be tried in US courts.

Washington was pushing countries in the region to prosecute captured pirates locally and was working to "help bring some political and economic stability to Somalia," said State Department spokesman Robert Wood.

"It's not easy. If it were easy, we would've been able to do so by now."

US officials were rattled by Monday's mortar attack in Mogadishu against Democratic Representative Donald Payne, who emerged unscathed hours after a pirate chief threatened to target Americans to avenge his men.

"This matter will lead to retaliation and we will hunt down particularly American citizens travelling our waters," pirate commander Abdi Garad told AFP by phone from the pirate lair of Eyl. "They (should) expect no mercy from us."

Merchant shipping companies must consider "armed security detachments" to safeguard their vessels, the commander of US naval forces in the region, Vice Admiral William Gortney, told US media from Bahrain Monday.

Gortney cited another ship that thwarted a boarding party by stringing barb wire along a vulnerable access point.

Democratic Representative Joe Sestak, a retired three-star admiral with extensive counter-terrorism experience, said ships may need "enhanced non-lethal" defenses that use heat or piercing sounds.

But "I'm real hesitant to say we should arm our (civilian) crews," Sestak told AFP, warning of escalating firefights between poorly trained sailors and pirates with "rocket-propelled arsenals."