Washington (CNN) Three of the Navy's nuclear-powered attack submarines are "not certified to dive today" due to maintenance delays caused by overcrowded shipyards, officials revealed Wednesday.

During a Senate Armed Services Committee hearing, Vice Chief of Naval Operations Adm. William Moran told lawmakers that one of the submarines, the USS Boise, will finally enter a shipyard in January after being out of service for four years.

Two additional attack submarines are currently not operational and will go into dry dock in the new year for repairs, according to Moran.

The Navy has been working to address its backlog of submarines, with some forced to wait years for maintenance, and has leaned on using private shipyards, in addition to public ones, to help speed up the process.

But while the service has taken steps to address maintenance challenges that have at times sidelined a significant chunk of its submarine fleet, Navy officials testified Wednesday that more work is needed.

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