Prime Minister, Stefan Löfven, warned that such incursions presented "enormous risks" for those involved and that Sweden would defend its borders "with all available means".

On 17 October the Scandinavian country launched its biggest submarine hunt since the cold war after witnesses reported seeing an underwater craft in the archipelago that extends from Stockholm into the Baltic Sea.

It called off the search after a week, saying the vessel had probably escaped into the Baltic, a news report by Prensa Latina News Agency said.

Löfven said the length of the Swedish coastline made it difficult to monitor but promised to strengthen the country's ability to identify intruders.(KH)