The Swedish corvette Visby is seen in the search for suspected "foreign underwater activity" at Mysingen Bay, Stockholm, Oct. 21, 2014. Reuters

Sweden and Poland, both concerned by increased Russian military activity in the Baltic Sea, have signed a military cooperation agreement during a meeting of their defense ministers in Warsaw, according to a report from the Local, a European online news site. The military agreement is one of many that Sweden has signed with its neighboring countries over the last year as it looks to compensate for not being part of NATO.

"Once a sea of peace, the Baltic has become a sea of danger," Polish Defense Minister Tomasz Siemoniak told reporters at a joint press conference in Warsaw Monday with his Swedish counterpart, Peter Hultqvist. Hultqvist said Russia’s presence in the region has prompted Stockholm to boost military spending by 11 percent over the last five years, and build better cooperation with NATO members and other countries not attached to the alliance.

The new relationships with Sweden, which is located directly across from Poland on the Baltic Sea, joins cooperative agreements with Denmark and Finland that Stockholm made earlier this year. Sweden already had pacts with Norway and Iceland as part of the Nordic Defense Cooperation. The Swedish government, traditionally neutral, said it plans to increase its cooperation with NATO this year, but stopped short of committing to joining the alliance as a full member. Poland has been a member of the alliance since March 1999.

The deal comes just a day after a new poll showed that more Swedes were in favor of joining NATO than against, demonstrating a shift in public opinion. A total of 41 percent said that they were in favor of joining the 28-member alliance, while 39 percent said they were against it and 20 percent were uncertain.

Most political parties in Sweden have traditionally opposed NATO membership, but earlier this month the center-right four-party group currently in opposition to the Social Democrat-Green coalition government decided to favor NATO membership.