Media playback is unsupported on your device Media caption USS Fitzgerald damaged by the crash

A major search operation is under way for seven missing US sailors after their ship collided with a merchant vessel off the coast of Japan.

Several Japanese and US boats and aircraft have been deployed, with the search continuing into the night.

An investigation into how the USS Fitzgerald collided with a Philippine-registered container ship in the early hours of Saturday morning has begun.

The ship's commanding officer was among three people aboard to be hospitalised.

The incident took place some 56 nautical miles (104km) south-west of the Japanese port city of Yokosuka, at about 02:30 local time on Saturday (17:30 GMT Friday).

The Navy ship's starboard side was heavily damaged in the collision, but the vessel later managed to slowly return to its base in Yokosuka, with the assistance of US navy tug boats.

USS Fitzgerald crash: In pictures

What happened?

The collision occurred near Yokosuka, which is home to the US 7th Fleet that comprises up to 80 submarines and ships, including the USS Fitzgerald.

It is unclear where the 154-metre (505ft) guided missile destroyer ship was heading at the time.

Image copyright AFP Image caption The US destroyer was guided back to base by US Navy tug boats

The ACX Crystal, a 222-metre (730ft) Filipino-flagged container ship, was travelling between the Japanese cities of Nagoya and Tokyo.

Marine traffic records suggest the ACX Crystal made a sudden U-turn roughly 25 minutes before the collision with the USS Fitzgerald. It is not clear why it changed course.

Marine traffic records suggest it was travelling at 14.6 knots (27km/h) at the time of the collision.

Image copyright MarineTraffic.com Image caption The ACX Crystal appeared to make a U-turn before the collision, before rapidly adjusting course and setting off to Tokyo

Marine traffic records for the USS Fitzgerald are not publicly available.

How bad are the damage and injuries?

Japan's coastguard is co-ordinating the search with US teams for the seven missing crew members, who have not yet been publicly identified. It has sent half a dozen vessels, a number of aircraft and rescue personnel to the scene.

The USS Dewey escorted the Fitzgerald back to Yokosuka, and a US P-8 Poseidon aircraft has joined search and rescue efforts, the 7th Fleet said in a statement.

It is not yet known where the missing sailors were when the collision happened.

The US navy says several compartments were severely damaged - including the ships radio room, and two berthing compartments, the BBC's Rupert Wingfield-Hayes reports from Tokyo.

Some of the seven missing sailors may have been asleep in those compartments when the collision happened, our correspondent adds.

Image copyright Reuters Image caption The collision left a huge gash down the right-hand side of the ship

Image copyright EPA Image caption The damage to the ACX Crystal was less serious than to the Navy ship

The Navy ship's commanding officer, Cdr Bryce Benson, is in a stable condition after being transferred by helicopter to US Naval Hospital Yokosuka, according to Navy officials. Two others were flown to hospital, and were said to be awake and under observation.

The ACX Crystal is 223m long and capable of carrying nearly 40,000 tonnes, much bigger than the 154m USS Fitzgerald.

It sustained lighter damage to its port bow, and has now reached Tokyo.

The Associated Press agency said there were no injuries reported among the 20 Filipino crew members on board the container ship.

What is the fallout?

Relatives of those on board the USS Fitzgerald have been posting on the US 7th Fleet's Facebook page, requesting information on their loved-ones. An information hotline has been set up.

In a statement, 7th Fleet commander Vice Admiral Joseph P Aucoin was quoted as saying "this has been a difficult day".

"I am humbled by the bravery and tenacity of the Fitzgerald crew. Now that the ship is in Yokosuka, I ask that you help the families by maintaining their privacy as we continue the search for our shipmates."

US President Donald Trump tweeted: "Thoughts and prayers with the sailors of USS Fitzgerald and their families. Thank you to our Japanese allies for their assistance."

The collision took place in waters that get heavy traffic and that have experienced prior collisions, Japanese broadcaster NHK said. Ships travelling to and from Tokyo, as well as crossing the Pacific in both directions, pass through the bay where the crash happened.

NHK said that an association of commercial ship captains is calling for the creation of east-west lanes in the area to avoid collisions.

One of the most advanced warships in the world - Rupert Wingfield-Hayes, BBC News, Tokyo

The area where the collision took place is extremely busy, with shipping heading in and out of Tokyo bay.

However, the USS Fitzgerald is one of the most advanced warships in the world - with highly sophisticated radars systems.

At the moment the focus is on the search for the missing sailors. Later will come the investigation into how one of the world's most sophisticated warships failed to avoid colliding with a 30,000-tonne cargo ship on a calm, clear night.