The U.S. Coast Guard is trying to alert boaters in Virginia's Elizabeth River to the possible presence of a 30 to 40-foot whale. The whale was spotted on Monday deep up the Elizabeth River, a tidal estuary near Hampton Roads that feeds into the Chesapeake Bay.

The whale's strange behavior has officials worried that it might be sick and disoriented. The whale has yet to be identified, but Mark Swingle, director of research and conservation at the Virginia Aquarium & Marine Science Center, told the Virginia-Pilot that it may be a sei whale.

Brook Smith, captain of the sailboat, American Rover, was one of several on the water to spot the whale in the last several days. He told the local Norfolk paper that he and passengers on his sailboat watched it swim in circles for about 20 minutes on Friday afternoon.

"It seemed to be kind of swimming in circles -- no particular direction," Smith said. "I'm not a whale expert, but it looked ill."

Swingle also said the whale was likely sick, telling the local paper: "It's definitely acting a little odd."

Sei whales are an endangered species that typically stick to deep ocean waters many miles from the coast. They are rarely seen.

The Elizabeth River is heavily trafficked by commercial vessels and is host to a variety of military operations. The Coast Guard put out a warning to commercial and recreational boaters to steer clear of the whale if they spot it, and to refrain from feeding it.