WASHINGTON — The Somali pirates who took a US merchant captain hostage for five days were heavily armed but inexperienced youths, US Defense Secretary Robert Gates said Monday, adding that the hijackers were aged 17 to 19.

The pirates who kidnapped Captain Richard Phillips, three of whom were killed by US Navy snipers Sunday, were "untrained teenagers with heavy weapons," Gates told a group of 30 students and faculty members at the Marine Corps War College in Quantico, Virginia.

"There is no purely military solution to" piracy in the region, he added.

"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."

A Pentagon spokesman confirmed Gates' quotes, which were first reported in the American Forces Press Service.

President Barack Obama on Monday said the United States was resolved to hold those who prey on merchant shipping accountable.

Earlier, he pledged to combat the rise of piracy off the coast of Somalia, after naval forces rescued Phillips from the small lifeboat where he was being held, surrounded by US warships.

"We remain resolved to halt the rise of piracy in this region," said Obama.

"To achieve that goal, we must continue to work with our partners to prevent future attacks, be prepared to interdict acts of piracy and ensure that those who commit acts of piracy are held accountable for their crimes."

Phillips, who commanded the US-flagged cargo ship Maersk Alabama, was held captive after the American crew on Wednesday fought off the pirates' attempt to capture it.