Sweden has proof that a small foreign submarine was operating illegally in its waters last month, the head of Sweden's armed forces, General Sverker Goransson.

"The military can confirm that a small U-boat breached Sweden's territorial waters. We can exclude all alternative explanations," Goransson said.

He said Sweden had not been able to identify which country was behind the intrusion.

Earlier this week the Svenska Dagbladet daily reported that the Swedish military has become even more convinced that at least one small foreign submarine operated in Swedish waters just off Stockholm last month.

Then reports of a mysterious underwater vessel prompted a hunt for what was believed to be a Russian submarine. More than 200 troops, stealth ships and helicopters scoured  the waters of the Baltic sea in the area of the Stockholm archipelago, but found nothing specific.

The submarine's presence was picked up by military sensors, Goransson said. Supporting evidence, he said, included a picture showing a bubble pattern typical of a diving submarine and a sonar image of tracks on the sea floor.

Prime Minister Stefan Lofven said intrusion into Swedish territorial water was unacceptable and that Sweden would bolster its capabilities in detecting and identifying such activity.

"We will defend Sweden's territorial integrity with all available means," Lofven said.

Meanwhile, also on Friday, Sweden and seven northern European nations have pledged closer cooperation in an effort to counter a recent increase in Russian military activity in the region, reports Deutsche Welle.

The defence ministers from the Baltic and Nordic states (Sweden and Finland are not members of NATO), as well as Britain, are holding military talks in the Norwegian capital of Oslo Thursday and Friday.

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