A U.S. Navy destroyer collided with a Philippine-flagged merchant vessel off the coast of Japan early Saturday morning local time, injuring sailors and causing significant damage to the U.S. ship.

Seven of the USS Fitzgerald's sailors were unaccounted for as of 9:30 p.m. Washington time. Chief of Naval Operations Adm. John Richardson tweeted that another three were medevaced, including ship's commanding officer, Cmdr. Bryce Benson. The ship's executive officer, Cmdr. Sean Babbitt, is now in command, according to Navy spokesman Rear Adm. Robert Durand.

The collision occurred about 56 nautical miles southwest of Yokosuka, Japan, and also involved the container ship ACX Crystal, a vessel more than four times the size of the destroyer. The Navy's 7th Fleet reported that some spaces were flooded, yet the destroyer was operating under its own power and is heading back to Yokosuka at about 3 knots.

The 7th Fleet said the USS Dewey, medical assistance and two Navy tugs are being dispatched as quickly as practicable to provide assistance. Naval aircraft are also being readied.

Japanese Coast Guard vessels and tugs from the Navy's Fleet Activities Yokosuka are assisting, along with the destroyer Dewey.

"Right now we are focused on two things: the safety of the ship and the well-being of the Sailors," said Adm. Scott Swift, commander of the U.S. Pacific Fleet. "We thank our Japanese partners for their assistance."

Aerial footage posted on a Japanese news site showed significant damage on the starboard side of the destroyer's superstructure above and below the waterline.

"As more information is learned, we will be sure to share it with the Fitzgerald families and when appropriate the public," Richardson wrote in a Facebook post. "Thank you for your well wishes and messages of concern. All of our thoughts and prayers are with the Fitzgerald crew and their families."