The U.S. Coast Guard said Wednesday it had not identified the source of underwater noises detected by sonar in the search for a missing submersible near the wreck of the Titanic.

"We don't know what they are, to be frank with you," said Captain Jamie Frederick regarding the sounds that had raised hopes the five people onboard are still alive.

"We have to remain optimistic and hopeful when you're in a search and rescue case," he told reporters in Boston.