The U.S. Department of Defense says the U.S. is doing all it can to assist South Korea in rescue efforts for the missing crew of the sunken naval boat, the “Cheonan.”

Pentagon spokesman Geoff Morrell said that it has yet to be determined what led to the sinking of the 12-hundred-ton ship but that officers from the U.S. Navy’s Seventh Fleet based in Japan have come to assist in the search for the 46 of 104 sailors that remain unaccounted for.

Morrell also noted that Joint Chiefs of Staff Chairman, Admiral Michael Mullen, placed a call to his South Korean counterpart, General Lee Sang-hee, and expressed his sympathies for the missing sailors and their families.