US sends warship
Early on Thursday, the US Navy warship Bainbridge arrived on the scene and was near the 17,000-tonne Maersk Alabama as crew members negotiated with the pirates for the return of the captain.
Half a dozen other ships that had been patrolling the area, but were hours away when the hijacking occurred, were heading to the scene.
The pirates had earlier seized the ship, but the unarmed crew managed to retake the vessel as the pirates got into a lifeboat with the captain.
The hijacking was the first involving an American crew off the Horn of Africa.
Kevin Speers, a spokesman for Maersk Line, the US subsidiary of Denmark's AP Moller-Maersk which owns the ship, confirmed that the crew had regained control of the Maersk Alabama after the pirates left the ship with the captain.
He added that the crew were unharmed.
Colin Wright, who identified himself as a third mate on the ship, told the Associated Press news agency by satellite phone that the crew were trying to negotiate the hostage's release.
Among the ship's cargo are 400 containers of food aid, including 232 containers belonging to the UN World Food Programme that were destined for Somalia and Uganda.
The Maersk Alabama hijacking comes just days after armed men took control of a British-owned ship, and the seizure of three other ships over the weekend.
There had been a relative lull in the number of hijackings in the first three months of 2009, with eight ships overrun by pirates in the Gulf of Aden, a major shipping route between the southern and northern hemispheres.
Somali pirates last year seized dozens of vessels close to the Somali coastline in their attempt to secure ransom payments, taking hundreds of sailors hostage.
The drop in the number of successful pirate attacks in the past three months was largely due to the increased presence of foreign navies in the area.
But the pirates have begun looking as far as the Seychelles in order to find unguarded ships to attack.
The gangs use speedboats launched from "mother ships" to give them an advantage over foreign navies patrolling the shipping lanes.
Pirates last year seized a Saudi supertanker containing $100m worth of crude oil.
The Sirius Star and its 25 crew were freed in January after $3m was dropped on to its deck.
Last September, a Ukrainian cargo ship carrying 33 Soviet-era T-72 tanks was also hijacked.
It was released five months later after $3.2m was reportedly paid as a ransom.