A US warship and negotiators from the Federal Bureau of Investigation were called in to help on Thursday, one day after a U.S.-flagged cargo ship was attacked by pirates off the coast of Somalia.

The Pentagon said the USS Bainbridge, a guided-missile destroyer, has arrived on the scene and was closely watching the situation.

FBI negotiators also were called in to help facilitate the release of the cargo ship captain still held by pirates, said FBI spokesman Bill Carter.

Captain Richard Phillips was being held on a lifeboat that was believed to be near the cargo ship Maersk Alabama.

Kevin Speers, a spokesman for the ship's owner, the Norfolk, Virginia-based Maersk Line Ltd, a subsidiary of Denmark's A.P. Moller-Maersk, told reporters Phillips has not been harmed.

"The safe return of the captain is our foremost priority," Speers said.

He also said the US Navy "is in command of the situation."

"We are in regular contact with the Maersk Alabama. The ship remains at a safe distance as instructed by the Navy," Speers said," We are coordinating with the Navy and all the governmental organizations involved in this crisis."

The Maersk Alabama, with 20 crew members aboard, was on its way to the Kenyan port of Mombasa with relief food aid when the pirates attacked it Wednesday.

After the attack, the crew managed to retake control of the ship and forced the four attacking pirates off of the vessel.

They also briefly held a pirate but the captain was held hostage by pirates in a lifeboat.

Ken Quinn, the ship's second mate, told the CNN in a telephone interview that the crew had tried to swap the pirate for their captain but other pirates continued to hold Phillips after their prisoner was released.

 "So now we're just trying to offer them whatever we can, food, but it's not working too good," Quinn said.

He said the pirates were armed with Kalashnikov assault rifles but the crew carried no weapons.

It was the first time in recent history that pirates targeted a U.S.-flagged ship.

The ship was some 450 km off the coast of Somalia, a distance that used to be considered safe from pirate attacks. The closest US warship at the time of the hijacking was 555 kilometers away and was unable to respond.

The US Navy several days ago warned ships in the area that pirates were increasingly operating farther and farther offshore.