Covering a surface area of 77 square kilometres (30 square miles) in both Swedish and Finnish waters, the spill was first detected on Wednesday in the Bothnian Sea.
"What the spill consists of is still not clear but it is not mineral oil, and there is currently no immediate threat of landfall," the coast guard said in a statement. It said it had mapped the spill using planes and also collected samples, adding that it would not be able to comment on which measures to deploy until after the samples had been analysed.
A preliminary investigation into environmental crimes has also been launched.
"Among other things, it is being investigated which ships have been in the area and what cargo they have had," the coast guard said.
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