A hugely embarrassing moment for the Russian Navy - if the Swedes can prove one of their submarines has been photographed off the coast of Stockholm.

It looks like Russia's been caught gathering intelligence and surveillance after a mystery submarine was photographed off the coast of Stockholm.

Swedish vessels continue to look for the craft - around 200 troops, several stealth ships, minesweepers and helicopters have been scouring the sea around islands about 30 miles east of the country's capital.

Sky News defence analyst Peter Roberts says Sweden has invested a huge amount in anti-submarine warfare capability as this has been a problem for many years, and it's clear what the country will do.

"And that's to search for it, to see if they can find it and then drive it from their waters to make an example of it and then to make the correct way of notifying Russia that they found of their top submarines inside their own waters."

Peter Roberts says the last time the Russians penetrated their territorial seas, in 1981, it took some time to drive the sub out.

Roberts adds Russia has diesel-operated submarines that are made, produced and trained for this sort of task.

"With photographs being taken, the captain is not going to have a pleasant homecoming when he goes back to his base.

"Russia is probably the only nation that could do this in terms of subbing operations."

Peter Roberts says over the last 50 years when the Russians have perpetrated Swedish territorial waters, the Swedish have searched for it to see if they can make an example of it.