Four other ships, including the Amorella, another passenger ferry carrying 1,313 people, were still stuck.

The ferries, nearly all transporting passengers between Sweden and Finland, had run into trouble just outside the Stockholm archipelago, made up of more than 20,000 islands, Lindvall said.

"They got caught outside the archipelago, where there is moving  ice. It's hard to navigate," he said, adding that he had not seen a situation with so many ships stuck at once since the mid-1980s.

Passengers safe

But the Viking Line ferry company insisted "there was at no time any danger to the passengers".

Jan Kaarstroem, the Viking Line chief executive, insisted in an interview with the TT news agency that his company's ferries were well equipped to handle ice and that all the passengers were safe.

Three ice breakers are in the area where the ferries are stuck and are expected to be freed by midday Friday.

Most of the commercial vessels had got stuck in the narrow Bay of Bothnia, where the ice is thicker, and around the autonomous Aaland islands.

Sweden has suffered an unusually harsh winter this year, with temperatures across the country almost continuously well below freezing since December.