Denmark’s National Maritime Authority issued a warning on Thursday for ships to avoid sailing through a key part of the Great Belt Strait, one of the world’s busiest sea lanes, due to a risk of “falling missile fragments.”

The Great Belt strait is the main maritime access to the Baltic Sea. A naval exercise was ongoing in the area. It was not immediately clear if the warning was linked to the military maneuver.

“Risk of falling missile fragments in the area... ships are requested not to pass through the area, (and to) lower their anchor if necessary,” the authority said in a brief message.

The Danish navy did not immediately respond to a request for comment. The Danish Maritime Authority also could not immediately comment.