Civilian shipping evacuated off Stockholm as helicopters and navy search for vessel that experts think is Russian midget sub

Swedish armed forces widened the hunt for a suspected submarine on Monday, with helicopters, naval vessels and some 200 military personnel combing the Stockholm archipelago for “foreign underwater activity”.

As the search expanded, the Swedish military warned civilian shipping to evacuate the search zone, and non-essential air traffic was banned from the area.

An indistinct picture released by the Swedish military shows a vessel apparently breaking the surface 30 miles east of Stockholm. According to the military, there have been three sightings of the craft since Friday.

The mystery vessel is more likely to be a midget sub of the kind used by Russia’s Spetsnaz (special forces) rather than a large nuclear one, according to defence officials and analysts. They said that it could be a Triton or Piranha – both midget submarines used to by military divers for special missions.

A large submarine would have been easier to find by now, the experts said, adding that Russia would not risk such boats in the narrow and shallow waters of the Swedish archipelago.

Russia issued a statement denying that any of its craft had violated Swedish sovereignty and suggested it could be a submarine from the Netherlands engaged in exercises in the Baltic. An unnamed Russian official was quoted by a Russian news agency as saying that Sweden could save the money it was wasting on the search by seeking an explanation from the Netherlands.

Dutch defence officials laughed at the suggestion. The Netherlands defence ministry said: “We participated in an exercise with Sweden with several ships, but it ended last week [on] Thursday.” It added: “[A] Dutch submarine is not involved.”

The Dutch ministry said the submarine, the Bruinvis, continued to Estonia where it had been at anchor in Tallinn.

Sweden had made no request to the US, Britain or Nato for assistance, said Therese Fagerstedt, a Swedish military press officer. She did not accuse Russia, saying there had been no confirmation yet of which country was involved.

A Nato official said the Swedes would not be dedicating so many assets to the search unless they were confident there was a foreign vessel in its waters.

“It is probably a small submersible that could poke around. It would not be a big sub in such a closed area,” he said. Russia’s nuclear fleet was designed for deeper waters than the Baltic, he added.

The official noted that there had been an increase in Russian military activity in the Baltic since the Ukraine crisis flared up earlier this year, particularly Russian fighter jets close to Swedish air space.

Estonia has stepped up surveillance of its territorial waters, with the border guard looking out for “potential anomalies”, a spokesman, Priit Pärkna, said.

Igor Sutyagin, a Russian specialist at the Royal United Services Institute in London, said he thought it was quite possible it was a Russian submersible.

He suggested it could be a Triton, which can carry two divers, is used by the Spetsnaz and is small enough to be hidden in a van, or a Piranha, which can carry four to six people. Russia only built two Piranhas, which is an almost totally silent submarine. One of the two was based with the Russian Baltic fleet.

Sutyagin noted that such midget subs were often launched from a hidden compartment in a ship and noted that there had been unusual movements by a Russian-owned oil tanker in the area. The tanker’s owners denied any involvement.

Oscar Jonsson, a Swedish researcher in Russian warfare at King’s College London, said: “The picture shows something that looks similar to a Triton in surface mode but we do not see much of it.”

He noted that the Swedish military in its press conference had stressed that it was not just one incident but that there had been a pattern of increased activity by an unnamed foreign country in the area.

Russia has stepped up military flights close to Swedish airspace, a move interpreted by Sweden and neighbouring Baltics states as an attempt to put pressure on it, in particular to dissuade Stockholm from any notion of joining Nato.

A submarine, unlike aircraft, would not be used to send a political signal but for purely military purposes, such as gathering intelligence, a naval source said.

Rear Admiral Anders Grenstad, of the Swedish navy, said on Sunday: “It could be a submarine or a smaller submarine. It could be divers using some form of moped-like underwater vehicle and it could be divers that don’t have any business on our territory.”

Sweden sold its submarine-hunting helicopter equipped with sonar to US firefighters in 2008 and the replacements have not yet arrived, making the hunt more difficult, according to analysts.