But the ice rarely gets thick enough in the Stockholm archipelago to trap powerful passenger ferries.

'Unfortunate wind'

The Regal Star, a cargo ship with 56 people on board, was also set free early Friday, while three other ferries that were trapped in the ice were able to break free on Thursday.

About 26 cargo ships were awaiting help from ice breakers in the northern parts of the Baltic Sea, but the situation was under control, officials said.

The maritime administration said the ships had ignored warnings about the icy conditions.

"Normally we can handle this type of obstacle," Viking Line CEO Jan Karstrom told SVT.

"But in this case the wind is unfortunate. It's blowing toward land and it means that [the ice] is packed more and more against land."

Johny Lindvall, who manages the maritime administration's ice breaker service, said 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.

"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.

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.