A US destroyer has arrived back in its home port after colliding with a container ship four times its size, leaving seven sailors missing.
The USS Fitzgerald, flanked by two tug boats and other support vessels, was listing to its right as it slowly approached Yokosuka Naval Base, south of Tokyo.
Crew members from the destroyer USS Dewey had helped stabilise the damaged destroyer after it collided with a container ship off the coast of Japan, leaving at least three sailors injured, in addition to the seven missing.
Family members were frantically seeking news, appealing via social media for calls from sailors aboard the ship as it slowly made its way to its home base.
Rescuers were searching for seven sailors who were thought to have been thrown into the sea or possibly trapped inside damaged sections of the destroyer, said Japanese coastguard spokesman Yoshihito Nakamura.
No one on the container ship was reported injured.
Footage from the Japanese TV network NHK showed navy crew members working to pump water from flooded sections of the crushed mid-right side of the USS Fitzgerald.
The ship's captain, Commander Bryce Benson, was airlifted to the US Naval Hospital in Yokosuka and was in a stable condition with a head injury, the US navy's 7th Fleet said in a statement.
Two other crew members suffered cuts and bruises and were evacuated, it said.
The Fitzgerald had limited propulsion and a US defence official said there was flooding in three compartments.
Most of the more than 200 sailors aboard would have been asleep in their berths, some of which were reportedly flooded.
The navy said that the collision occurred 56 nautical miles south-west of Yokosuka, home to the 7th Fleet.
"Right now we are focused on two things: the safety of the ship and the well-being of the sailors," said Admiral Scott Swift, commander of the US Pacific Fleet.
An investigation was under way to determine what caused the night-time collision between the destroyer and the container ship.
Conditions at the time were clear, though Yutaka Saito of the coastguard said the area is particularly busy with sea traffic.
The Japanese coastguard said it received an emergency call from the Philippine-registered container ship ACX Crystal at around 2.20am that it had collided with the Fitzgerald.
Relatives of crew members were awaiting news of their loved ones.
"Just heard the sweetest voice and saw a wonderful face. He's okay. Thank you all for the prayers," tweeted Rita Schrimsher of Athens, Alabama, after speaking with her 23-year-old grandson, Jackson Schrimsher, via Facetime.
"It could have been worse, so we're grateful," she said.
The Philippine ship is 29,060 tons and is 222 metres long, the coastguard said, much larger than the 8,315-ton naval destroyer.
Aerial television news footage showed its bow on the left side was dented and scraped, but it did not appear to have suffered any major structural damage.