In a scenario reminiscent of the 1970s or 1980s, Sweden is currently searching for an alleged Russian submarine within its territorial waters. Though Russia denies the allegation that one of its vessels is inside Swedish territory, the Swedish navy has been searching for an unknown vessel in its waters for the past four days.

Sweden has summoned minesweepers, helicopters, the stealthy Visby-class as well as 200 men to search for the submarine, according to AFP, some 30 miles of the capital Stockholm. They are searching for “foreign underwater activity” because of a tip off by “credible sources,” the military said.

“I have decided to increase the number of units in the area -- units with specialized sensors," Commander Jonas Wikstroem told reporters at a press conference in the capital. "We still judge that the information we received yesterday was very trustworthy."

Sweden’s recently-appointed defense minister, Peter Hultqvist, turned down commenting on the current situation but alluded to Russia’s recent excursions into Swedish territory.

"What's been happening in the Baltic Sea, including airspace incursions, shows that we have a new, changed situation," he told the newspaper Svenska Dagbladet.

On Sunday, to corroborate their suspicions, Sweden released a photo of a vessel “surrounded by white ripples,” according to Reuters. The witness who took the photograph said it submerged soon after.

"It could be a submarine, or a smaller submarine," Rear Admiral Anders Grenstad said in a news conference. "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."

Citing unnamed but reliable sources, the daily Svenska Dagbladet reported that Sweden became aware of the intrusion when it intercepted a message in Russian on an emergency frequency. However, Adm. Grenstad said the military had not received information on a distress signal.

Nonetheless, the Russian defense ministry denied an emergency call happened, saying that Russian navy vessels were "performing tasks ... according to plan,” according to the BBC.

"There has been no irregular situation, let alone emergency situation, involving Russian navy vessels," the ministry spokesman said.