The US navy says seven crew members are missing and at least three injured after a destroyer, the USS Fitzgerald, collided with ACX Crystal, a Philippine-flagged merchant vessel off the coast of Japan.

The Japanese coastguard said the Fitzgerald was experiencing some flooding, but was not in danger of sinking, while the merchant vessel was able to sail under its own power.

The coastguard said none of the 20 crew members on board the merchant vessel were injured.

Three aboard the destroyer had been medically evacuated, including the ship's commanding officer, Commander Bryce Benson, who was reportedly in stable condition after being airlifted to the US Naval Hospital in Yokosuka.

The other two injured were transferred to the hospital for lacerations and bruises, while other injuries were being assessed, the US navy statement said.

The Fitzgerald was operating under its own power, "although her propulsion is limited".

It was unclear how the collision south of Tokyo Bay happened.

"Once an investigation is complete then any legal issues can be addressed," the US navy's Seventh Fleet spokesman said.

'Grounds for fear'

The Fitzgerald and the Japanese coastguard were searching for seven missing sailors.

Speaking to Al Jazeera from Tokyo, Michael Penn, a journalist with the Shingetsu news agency, said: "As hours and hours have gone by [and] it was in the middle of the night that they went overboard, there are strong grounds to fear for them."

The Fitzgerald suffered damage on its starboard side, above and below the waterline, but the full extent of the damage was still being determined.

"The US navy ship is an Aegis destroyer, which has one of the most sophisticated radars in existence. So how such a ship was taken unaware and rammed by a merchant ship is very bizarre," Penn said.

The ACX Crystal is about three times the size of the warship USS Fitzgerald [Kyodo via Reuters]

The waterways approaching Tokyo Bay are busy with commercial vessels sailing to and from Japan's two biggest container ports in Tokyo and Yokohama.

The USS Dewey and two navy tugboats had been dispatched to provide assistance, the US navy said.

Japan's public broadcaster NHK showed aerial footage of the destroyer, which had a large dent in its right, or starboard, side. Images broadcast by NHK showed it had been struck next to its Aegis radar arrays behind its vertical missile launch tubes.

In May, the US navy's USS Lake Champlain collided with a South Korean fishing vessel, but both ships were able to operate under their own power.

Source: Al Jazeera and news agencies