TOKYO (Reuters) - Rescue crews searched into the early hours on Sunday for seven American sailors missing after a U.S. destroyer collided with a container ship in the pre-dawn hours off the coast of Japan.

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U.S. 7th Fleet Vice Admiral Joseph P. Aucoin said the search was continuing in a statement released nearly 24 hours after the USS Fitzgerald, an Aegis guided missile destroyer, collided with the much larger Philippine-flagged merchant vessel 56 nautical miles southwest of Yokosuka.

“It’s been a tough day for our Navy family. It’s hard to imagine what this crew has had to endure, the challenges they’ve had to overcome,” Aucoin said.

U.S. and Japanese aircraft and surface vessels continued the search after the Fitzgerald sailed into the port of Yokosuka south of Tokyo. Three aboard the destroyer were treated at the U.S. Naval Hospital, including ship Commander Bryce Benson.

It was not clear what caused the collision, which the U.S. Navy said occurred at about 2:30 a.m. local time (1730 GMT).

“Thoughts and prayers with the sailors of USS Fitzgerald and their families. Thank you to our Japanese allies for their assistance,” U.S. President Donald Trump said in a Twitter post on Saturday.

The Fitzgerald suffered damage on her starboard side above and below the waterline, the Navy said.

Japan's Nippon Yusen KK 9101.T, which charters the container ship, ASX Crystal, said in a statement it would "cooperate fully" with the Coast Guard's investigation of the incident. At around 29,000 tons displacement, the ship dwarfs the 8,315-ton U.S. warship, and was carrying 1,080 containers from the port of Nagoya to Tokyo.

None of the 20 crew members aboard the container ship, all Filipino, were injured, and the ship was not leaking oil, Nippon Yusen said. The ship arrived at Tokyo Bay later in the day.

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