The Swedish navy’s search for a ‘mystery foreign vessel’ in the waters near Stockholm continues for a fourth day, with ships, helicopters, and around 200 troops involved in the operation.

The military has reportedly asked civilian vessels to evacuate the area between the island of Nattaro and Danzinger Gatt as they try and close in on the unknown vessel in the Stockholm archipelago, made up of some 30,000 islands.

Early rumours on Friday were that the vessel was some form of Russian submarine, possibly a ‘midget sub’, that was lost, damaged or possibly replacing old underwater espionage equipment. However, the waters in which it has been spotted are well within Swedish territorial waters, and any foreign vessel in this location would be a violation of Swedish waters and would cause an international incident.

Russia has dismissed the notion that there is an “irregular situation” involving its ships, and claimed that a Dutch submarine in the area may have triggered the Swedish alert system, a claim quickly denied by The Hague.

The conflict in east Ukraine has resulted in increased alert levels about the possible threat from Russia in the Baltic region.

The vessel has been spotted three times since Friday in the waters near Stockholm, at Kanholmfjärden, Nämdöfjärden, and Jungfrufjärden.