Sweden has released an image of underwater tracks which it says is clear proof that a foreign submarine illegally entered its waters last month, in an event which triggered a national search mission reminiscent of the cold war.

The sonar image shows tracks on the sea floor, which the head of the armed forces says were left by the submarine. “The military can confirm that a small submarine breached Sweden’s territorial waters. We can exclude all alternative explanations,” General Sverker Göranson told a news conference.

He said Sweden was no closer to finding out which country was behind the incursion. Suspicion had fallen on Russia, but its defence ministry denied the allegation, dismissing the Swedish search operation as a “tragicomedy”.

In unusually stark language for the non-aligned country, the Swedish 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”. “Let me say this loud and clear, to those who are responsible: it is completely unacceptable,” he added.

On 17 October, Sweden began its biggest military operation since the end of the cold war, after a member of the public contacted the armed forces with a reported sighting of a submarine. The search was concentrated in the archipelago outside Stockholm, but was called off after a week after no trace of the vessel could be found.

More than 200 troops, battleships, minesweepers, and helicopters had been deployed to scour the vast area of water, about 30-60km from the Swedish capital. Specialists had likened the search to looking for a needle in a haystack.

The search was reminiscent of the worst days of the cold war, when Sweden ramped up military spending after a Russian nuclear-armed submarine became grounded on rocks in the south of the country in 1981.

For a decade, the navy conducted regular searches of Swedish waters in the hunt for submarines, sometimes dropping depth charges on suspect objects.

This latest development risks inflaming tensions between Europe and Russia. The British prime minister, David Cameron, is meeting Vladimir Putin, the Russian president, at the G20 summit in Brisbane, Australia, before a possible bolstering of sanctions against Russia if it does not stick to the terms of a ceasefire agreement with Ukraine.