Seven U.S. navy sailors were missing and at least two, including the captain, were injured after a U.S. destroyer collided with a merchant ship off the coast of Japan before dawn Saturday, the U.S. navy and the Japanese coast guard reported.

Footage from the Japanese TV network NHK showed heavy damage to the mid-right side of the USS Fitzgerald and a person in a stretcher being lifted to a helicopter.

The first, Cmdr. Bryce Benson, was taken to the U.S. Naval Hospital in Yokosuka and was in stable condition, the U.S. 7th Fleet said in a statement. It said another injured person was being medevac'd but gave no further details on the crew's injuries. Others hurt in the crash were being evaluated, it said.

The Fitzgerald had limited propulsion after suffering damage on the starboard side below the water line, and a U.S. defence official said there was flooding in three compartments.

It wasn't clear yet what caused the nighttime collision between the destroyer and the container ship four times its size. Most of the more than 200 sailors would have been asleep in their berthings, some of which were reportedly flooded. 

The navy and Japanese maritime defence vessels and the coast guard were working to stabilize the destroyer as it headed to shore, said navy chief Admiral John Richardson. 

The navy said that the collision occurred 56 nautical miles (103 kilometres) southwest of Yokosuka, which is home to the 7th Fleet. Footage showed the damaged ship moving slowly in calm seas, flanked by a tugboat. 

Search and rescue operations underway

"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 U.S. Pacific Fleet. 

The Japan coast guard said it received an emergency call from a Philippine-registered container ship ACX Crystal around 2:20 a.m. local time that it had collided with the Fitzgerald southwest of Yokusuka, Japan.

Takeshi Aikawa of the Japan coast guard said that so far no damage or injuries were reported on the container ship. NHK reported that the merchant ship had scratches on the left side of its bow.

Japan-US-Navy-Collision

The U.S. navy said the collision occurred 56 nautical miles southwest of Yokosuka, Japan, a city south of Tokyo that is home to a major U.S. naval base. (Kelsey L. Adams/U.S. navy/AP)

The Philippine ship is about 26,000 tonnes and is 220 metres long, the coast guard said.

The fleet said the USS Dewey, medical assistance, navy tugs and naval aircraft were dispatched. The Japan coast guard dispatched five patrol ships and an aircraft carrying medics to the site for search and rescue operations.