Pirates are believed to be holding Capt Richard Phillips
US crew members have recaptured their ship after it was hijacked by Somali pirates, but their captain is still being held hostage by the attackers.
The Maersk Alabama was taken by the pirates about 500km (311 miles) off Somalia's coast after a lengthy battle.
The crew later fought back and retook the ship, but the captain was captured by the pirates who fled in a lifeboat, crew members have told US media.
A US warship and other vessels are speeding towards the scene.
The cruise-missile carrying USS Bainbridge is among the ships the US Navy has despatched, officials told the Associated Press.
US Secretary of State Hillary Clinton said the government was following the situation very closely and urged the world to act to end the "scourge" of piracy.
US media have telephoned members of the ship's crew to get details of their struggle against the pirates.
Second mate Ken Quinn told CNN how the crew captured one of the pirates and kept him tied up for 12 hours.
As they attempted to negotiate the release of their captain, who has been named as Richard Phillips, they freed the captive attacker.
But the gang refused to free Capt Phillips.
"Right now they want to hold our captain for ransom, and we are trying to get him back," second mate Quinn said.
"So now we're just trying to offer them whatever we can - food. But it's not working too good."
He said the attackers had fled in a lifeboat and crew members were using radios to keep in contact with Capt Phillips.
In a statement, the ship's owners, Maersk, confirmed much of the sailor's account.
"The armed hijackers who boarded this ship earlier today have departed, however they are currently holding one member of the ship's crew as a hostage," Maersk said.
"The other members of the crew are safe and no injuries have been reported."
Upsurge in hijackings
The ship was first attacked by several pirate boats in the early hours of Wednesday.
It is not clear how many attackers were involved, but accounts from the sailors on the Maersk Alabama suggest that four boarded the vessel.
Maritime officials said the ship took all possible evasive action before it reported that the pirates had boarded.
Pirate attacks have been increasing rapidly in recent years - more than 130 incidents were reported in 2008, including almost 50 successful hijacks.
Pirates typically hold the ships and crews until large ransoms are paid by the shipping companies - last year the firms handed over about $80m (£54m).
After a lull earlier this year, this was the sixth ship seized off Somalia in the past week.
The attacks are threatening to destabilise one of the world's busiest shipping lanes.