The US Navy has promised a full investigation into a collision between a destroyer and a container ship off Japan that killed seven sailors.

The navy’s acting secretary Sean Stackley said “we are all deeply saddened by the tragic loss of our fellow shipmates”.

He also praised their colleagues for saving the USS Fitzgerald from further damage and bringing it back to port.

The container ship ACX Crystal with its left bow dented and scraped after colliding with the USS Fitzgerald in the waters off the Izu Peninsula (Hitoshi Takano/Kyodo News via AP)The container ship ACX Crystal with its left bow dented and scraped after colliding with the USS Fitzgerald in the waters off the Izu Peninsula (Hitoshi Takano/Kyodo News via AP)

Mr Stackley promised to “fully investigate” the cause of the collision at about 2.20am on Saturday between the Fitzgerald and a Philippine-flagged container ship four times its size, as many of the crew slept.

Navy divers recovered the bodies after the severely-damaged Fitzgerald returned to the The 7th Fleet’s home in Yokosuka, Japan, with assistance from tug boats.

The dead sailors were aged between 19 and 37.

Commander of the 7th Fleet Vice Admiral Joseph Aucoin described a harrowing scene as sailors fought to keep the ship from sinking.

Most of the damage was below the waterline, including a large gash near the keel, he said.

“The water flow was tremendous, and so there wasn’t a lot of time in those spaces that were open to the sea. And as you can see now, the ship is still listing, so they had to fight the ship to keep it above the surface. It was traumatic,” he said.

Vice Admiral Aucoin said one machinery room and two berthing areas for 116 crew members were severely damaged from the impact to the ship’s side.

Navy spokesman Lieutenant Paul Newell said the victims may have been killed by the impact of the collision or drowned in the flooding.

The Fitzgerald’s captain, Commander Bryce Benson, suffered a head injury in the collision and was airlifted to the US Naval Hospital in Yokosuka. Two other crew members suffered cuts and bruises and were also flown out by helicopter.

The container ship ACX Crystal goes through the Tokyo Bay after colliding with the USS Fitzgerald (Shunpei Ishii/Kyodo News via AP)The container ship ACX Crystal goes through the Tokyo Bay after colliding with the USS Fitzgerald (Shunpei Ishii/Kyodo News via AP)

Conditions were clear at the time of the collision, though the area is particularly busy with sea traffic.

The damage to the destroyer suggests that the container ship, the ACX Crystal, might have slammed into it at a high speed, raising questions about communication between the two vessels in an area where as many as 400 ships pass through every day, according to Japan’s coastguard.

Most congestion occurs in the early hours of the day, and fast currents make it a tricky area that requires experience and skill to navigate.

The ACX Crystal weighs 29,060 tons and 730 feet, much larger than the 8,315-ton destroyer.

The container ship’s left bow was dented and scraped, and accident investigators from the Japanese transport ministry found further damage below its waterline.

Footage from Japanese broadcaster NHK showed a sharp horizontal cut across the bow area, which looked like a shark’s mouth. Many scratches were also seen in the frontal area.

The container ship ACX Crystal with its left bow dented and scraped after colliding with the USS Fitzgerald (Hitoshi Takano/Kyodo News via AP)The container ship ACX Crystal with its left bow dented and scraped after colliding with the USS Fitzgerald (Hitoshi Takano/Kyodo News via AP)

Some ship trackers showed the container ship making a U-turn before the collision, a move that has raised questions about what happened.

The coastguard questioned crew members of the ACX Crystal and is treating the collision as a case of possible professional negligence, said Masayuki Obara, a regional official.

All of the ACX Crystal’s 20-member Filipino crew were safe, according to Japanese shipping company Nippon Yusen, which operates the ship.

A screen shows a news program reporting about a collision between a US destroyer and a container ship off Japan, in Tokyo (Koji Sasahara/AP)A screen shows a news program reporting about a collision between a US destroyer and a container ship off Japan, in Tokyo (Koji Sasahara/AP)

Japanese prime minister Shinzo Abe sent a sympathy message to US president Donald Trump, saying: “We are struck by deep sorrow.

“I express my heartfelt solidarity to America at this difficult time.”