Sweden's biggest submarine hunt since the dying days of the Soviet Union has put countries around the Baltic Sea on edge, with Latvia's foreign minister calling the incident a potential "game changer" in the region.

In a scene reminiscent of the Cold War, Swedish naval ships, helicopters and ground troops combed the Stockholm archipelago for a fourth day Monday for signs of a foreign submarine or smaller underwater craft that officials suspect entered Swedish waters illegally.

While Sweden hasn't said what foreign country they suspect — and Moscow denies involvement — the submarine search sent a chill through the Baltic Sea region, where Russian forces have been accused of a series of border violations on land, sea and air in recent months.