The Swedish Navy has expanded its search for a Russian submarine believed to be off the Stockholm waters.

Reports said that the Swedish navy had civilian vessels removed from the areas surrounding Danziger Gatt and Nattaro Island.

In an interview with the BBC, a Swedish source revealed that a submarine was spotted about 16 miles off the waters of Stockholm.

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However, the defense ministry of Russia has denied any reports claiming that one of its underwater vessels was in an "irregular situation," adding that it might have been a Dutch submarine that was spotted by the Swedish radar after performing its exercises in the area.

Meanwhile, the Dutch defense ministry opposed the Russian defense ministry's claim. According to BBC, quoting a ministry spokeswoman in The Hague, the submarine "was definitely not Dutch."

The search, which was on its fourth day, has involved the use of helicopters and ships.

A statement released by the Swedish military said it was responding to three "credible reports" saying "foreign underwater activity" have been spotted in the area.

Additionally, the ministry said it was not looking for an underwater vessel, but was conducting an intelligence operation.

The ministry also released a blurry photograph of an unidentifiable object near the Stockholm waters.

According to a Swedish official who refused to be identified, the Swedish navy has closed the airspace in the area and has prevented civilian ships and vessels to approach within 10 kilometers of the navy ships.

The official added that it could not have been a mistake in the submarine's navigation, since the Stockholm archipelago was hard to navigate.

When asked about his thoughts regarding why a submarine would enter the Swedish waters, he said it was a move to test the preparedness of Sweden.

"It's a way of testing our skills in detecting and locating," the official said.

TagsSweden, Russian submarine, Stockholm, foreign activity, underwater activity