"Since record keeping began in the '60s, we've never encountered anything like this before," ice-breaker Ulf Gulldne said.
Almost 70,000 square miles of the Baltic Sea was covered in ice March 29, a record for the time of year, TT/TheLocal.se reported Friday.
About half of the central and northern Baltic are frozen over and the ice is both thick and difficult to break through in the far north, experts said.
"I've never seen this much ice this late in the season," Karl Herlin, captain of the icebreaker Atle working off the coast of northern Sweden, said.
The Atle is freeing up a path through the ice for a ship making its way to the city of Lulea, one of between five to 15 ships that Atle has assisted every day in the past week, Herlin said.
The Swedish Maritime Administration said all its five ice-breaking ships are currently in service.
Southern parts of the Baltic Sea, normally clear of ice this time of year, retain some ice all the way past Stockholm, Sweden, because a prolonged spell of high pressure that arrived in early March caused new ice to form late in the season, experts said.
"The cold is unusually stubborn, as normally the ice would have started to melt by now," Torbjorn Grafstrom at the Swedish Meteorological and Hydrological Institute said.