Story highlights "This has been a difficult day," US Navy commander says

The incident occurred in a busy maritime zone

Tokyo (CNN) The search was on for seven US sailors missing after an American warship and a merchant vessel collided off the coast of Japan, as US forces and Japanese authorities worked to figure out why the vessels struck each other.

The 10,000-ton guided-missile destroyer USS Fitzgerald and the ACX Crystal, a 29,000-ton container ship flagged in the Philippines, collided early Saturday local time, officials said.

The destroyer suffered severe damage to its starboard side, while the container ship sustained light damage.

"This has been a difficult day," Vice Adm. Joseph P. Aucoin, commander of the US 7th Fleet, said.

Forces overnight intensified their search for the missing sailors and tried to get a handle on the cause of a crash in a busy maritime zone crossed daily by 400 to 500 ships.

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