The Swedish military has ramped up search efforts for a suspected Russian submarine off the coast of the capital of Stockholm, military officials announced Saturday.

After it emerged that the Swedish military was investigating a report of "foreign underwater activity" from a "credible source, Commander Jonas Wikstroem said he had "decided to increase the number of units in the area - units with specialized sensors."

Wikstroem would not confirm or deny that the search was for a distressed Russian submarine.

Since Friday, over 200 men, stealth ships, helicopters and minesweepers have been searching a stretch of the Baltic Sea about 50 kilometers east of the Swedish capital, although the military has not announced the results of the search thus far. Sweden has not seen a mobilization of this sort since the Cold War.

"We still judge that the information we received yesterday was very trustworthy," Wikstroem said.

Swedish daily Svenska Dagbladet reported that the military intercepted a distress call in Russian intended for a port in the city of Kaliningrad. Defense analysts have told local media that it is possible a foreign submarine may have been in the area to replace spy equipment, or monitor a Swedish naval exercise.

The operation is taking place amid rising fears in the Baltic countries over the Ukraine crisis and concerns about potential renewed Russian territorial aggrandizement. Russian President Vladimir Putin recently was quoted as saying, "if I wanted, Russian troops could not only be in Kyiv in two days, but in Riga, Vilnius, Tallinn, Warsaw or Bucharest, too."

Last week Finland accused the Russian navy of interfering with one of its research vessels in international waters.

"What's been happening in the Baltic Sea, including airspace incursions, shows that we have a new, changed situation," Swedish Defense Minister Peter Hultqvist told the Svenska Dagbladet. "Russia has made enormous military investments...with their increased strength they are training more, and that influences the security environment."

Sweden, which sought to destroy alleged Russian submarines near the end of the Cold War, has also been monitoring an increased number of Russian air force maneuvers over the Baltic Sea. Last month two Russian warplanes allegedly entered Swedish airspace. Foreign Minister Carl Bildt called it "the most serious aerial incursion by the Russians" in nearly 10 years.

bw/jm (Reuters, AFP,dpa)