Aerial television footage showed one sailor lying on a stretcher and a rescuer being pulled up to a helicopter that was hovering above the USS Fitzgerald, its right side partially crushed.

Three sailors were injured in the pre-dawn collision, including the guided missile destroyer's commanding officer Bryce Benson.

"Two sailors, in addition to the commanding officer, have been medically evacuated from USS Fitzgerald to US Naval Hospital Yokosuka for lacerations and bruises," the navy said.

"There are seven sailors unaccounted for; the ship and the Japanese Coast Guard continues to search for them."

The accident between the Fitzgerald and Philippine-flagged ACX Crystal happened around 2:30 am (1730 GMT Friday) off the coast off the Izu peninsula southwest of Tokyo, the US Navy and Japan's coastguard said.

The area is a busy shipping channel near major container ports in Yokohoma and Tokyo.

"The volume of ships is heavy in this area and there have been accidents before," coastguard official Yutaka Saito told Japan's public broadcaster NHK.

It was not immediately clear what caused the collision, but NHK said the massive 222-metre (730 foot) container ship made a sharp turn around the time of the accident.

Japan's coastguard, which is probing the incident, has sent five vessels, two planes and a team of specially trained rescue personnel to help in the search for the missing crew, a spokesman said.

The USS Dewey and two Navy tugboats were helping out in the search and rescue, the US military said, while Japan's Self-Defence Forces also joined the operation, sending three ships and several aircraft.

'So worried' 

The 154-meter Fitzgerald -- which was commissioned in 1995 and was deployed in the Iraq war in 2003 -- is based in Yokosuka and operates in the Pacific and the Sea of Japan (East Sea).

The accident happened 56 nautical miles (104 kilometres) southwest of Yokosuka, the navy said.

"My daughter is on the Fitzgerald," a parent wrote on the 7th Fleet's Facebook page.

"So worried. Just need to hear she is ok. Thinking of all of our sailors and their families!!"

US chief of naval operations Admiral John Richardson said in a statement: "As more information is learned we will be sure to share it with the Fitzgerald families and when appropriate the public.

"All of our thoughts and concerns are with the Fitzgerald crew and their families."

Video by Japan's public broadcaster NHK showed heavy damage to the right side of the Fitzgerald just ahead of the control tower, and that the ship had taken on water.

An orange hose was visible on the ship's deck, with many crew members seen checking the damage.

The destroyer, piloted by two tugboats, was sailing to Yokosuka.

"The USS Fitzgerald suffered damage on her starboard side above and below the waterline. The collision resulted in some flooding," the Navy said.

The Fitzgerald was still under its own power "although her propulsion was limited", the navy said. It is not in danger of sinking and is headed back to port on its own power.

The footage meanwhile showed relatively light damage to the ACX Crystal.

Twenty crew members were aboard the container and none was injured, another Japanese coastguard spokesman said, adding that the vessel was separately sailing back to a Tokyo port.

"We are now investigating the case as further details, including the weather conditions at the time, are not available," the spokesman said.

The ACX Crystal is a commercial container ship that, according to marine tracking websites, was on its way to Tokyo.

The ship carries a Philippine flag but is listed as owned by Japanese shipping giant NYK Line.

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