Sweden has confirmed that a small foreign submarine illegally entered its waters off the coast of Stockholm last month.

Military officials said the nationality of the intruders can not be confirmed, but several defense analysts say Russia remains the likely culprit.

Russian media has in turn responded by claiming that it was a NATO submarine, and accusing the Swedes of cowardice and deliberately keeping the nationality a secret.

Proof: An image of the seabed off the coast of Stockholm which was provided as evidence of foreign submarine activity at today's press conference

The Swedish Armed Forces' Commander-in-chief Sverker Göranson confirmed the violation during a press conference this morning, held with Prime Minister Stefan Löfven and Defense Minister Peter Hultqvist.

'The Swedish Armed forces can now confirm that a smaller submarine violated Swedish territory,' Göranson said.

'There is no doubt about this. We can rule out all other alternative explanations.'

Göranson added that the government is 'one-hundred per cent sure' of the violation, which he calls 'deeply serious'.

'It's impossible to confirm any nationality. But we can confirm the fact that it has been there,' Göranson said.

Sweden's Prime Minister Stefan Löfven, Defence Minister Peter Hultqvist, and The Swedish Armed Forces' Commander-in-chief Sverker Göranson arrive for a news conference in Stockholm where they confirmed that a small foreign submarine illegally entered its waters last month

Is this it? This grainy amateur photo showing a mysterious vessel in Stockholm's archipelago was released by the Swedish military during the hunt, the captions bottom right reads: 'Object moves at speed through the water. Bearing south. Heavy self-generated foaming'

Prime Minister Löfven warned that such incursions into Swedish territory presented 'enormous risks' for those involved and that Sweden would defend its borders 'with all available means'.

Should a nationality be confirmed at any point, it will be a major test for Mr Löfven, a former welder who has never even sat in parliament before becoming prime minister, and his Social Democrat-Green Party coalition government.

During the week-long hunt for the intruders, it was reported to have been a Russian intrusion into Swedish waters, and several defense experts maintain that this is the case.

Today, former foreign minister Carl Bildt made a thinly veiled hint at Russian responsibility, by tweeting: 'Swedish Armed Forces can’t identify nationality of intruding submarine. Looks like small green subs'

The 'small green subs' refers to the Russian annexation of Crimea, which began with 'green men' landing on the Ukrainian peninsula.

The leader of the Swedish Liberal Party, Jan Bjorklund, went even further, telling Aftonbladet, that there could be no other conclusion than that Russia was responsible.

Russian media was quick to hit back, with pro-Putin newspaper Rossijskaja Gazeta accusing NATO of being responsible for the intrusion, and the Swedish military of keeping schtum about it.

'So it is about their own - NATO. But to say that means admitting that the Russians are not responsible.

'But the Swedish military is clearly not able to take such a manly step. This is why Mr Göranson acts as if he had water in his mouth [a Russian saying meaning 'clamming up']

Any other logical explanation to his inability to announce which country the submarine was from, clearly does not exist,' writes Rossijskaja Gazeta.

Under attack: Although he Swedish government has confirmed an illegal breach of Swedish borders, the nationality of the vessel cannot be confirmed

Armed and ready: Prime Minister Löfven said Sweden would defend its borders 'with all available means'

Göranson said the military obtained evidence of the intrusion with its own sensors,and provided images of the seabed where trails left by the submarine could be seen.

Löfven noted that Sweden's coastline is as long as the U.S. East Coast, making it difficult to monitor, but promised to strengthen the country's ability to find and identify intruders.

'Let me say this, loud and clear, to those who are responsible: It is completely unacceptable,' Löfven said.

The Scandinavian country launched its biggest submarine hunt since the Cold War on October 17, after receiving eyewitness reports of 'underwater activity' in the archipelago that extends from Stockholm into the Baltic Sea.

The week-long hunt has involved more than 200 troops, stealth ships and helicopters but the military called it off after concluding that the vessel had probably escaped into the Baltic.

The military said then that it was likely that a foreign submarine or divers had been active in the archipelago, but fell short of concluding that there was absolute proof of an intrusion into Swedish water.

Sweden built up an anti-submarine force after a Soviet sub with nuclear weapons ran aground off its southern shores in 1981, but started dismantling the force as part of deep cuts in defense spending after the Cold War ended.