"Once a sea of peace, the Baltic has become a sea of danger," Polish Defence Minister Tomasz Siemoniak told reporters at a joint press conference in Warsaw with his Swedish counterpart Peter Hultqvist on Monday.
Hultqvist said the increased presence of Russian warships and warplanes in the Baltic Sea had prompted Stockholm to take "two strategic decisions".
One was to boost defence spending by 11 percent over five years, and the other was to reinforce cooperation with Nato as a whole, as well as with its individual members.
Sweden's foreign ministry on Friday summoned Russia's ambassador after Moscow threatened "retaliatory measures" if the Scandinavian country joined Nato.
The increased tensions come on the heels of Russia's 2014 annexation of Ukraine's Crimean peninsula and backing of separatist forces in the country's east.
According to Sweden's Security Service Säpo, the biggest intelligence threat against Sweden came from Russia last year.
Sweden has already moved to extend military cooperation with other neighbouring countries. In April, Sweden, Norway, Denmark, Finland and Iceland announced far-reaching plans which Hultqvist also described as “a direct response to aggressive Russian behaviour”.
The Nordic country has also said it plans to increase its participation in Nato exercises.
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Hultqvist's deal with Poland comes as a new poll suggests more Swedes are now in favour of joining Nato than against the idea, representing a rapid shift in public opinion.
A total of 41 percent told pollsters that they were in favour of seeking membership in the military defence alliance, 39 percent said they were against it and 20 percent were uncertain.
Political scientist and Social Democrat Ulf Bjereld told the TT newswire late on Sunday that he believes two factors have influenced Swedes' change of heart: a perceived threat from Russia and outgoing Supreme Commander of the Armed Forces Sverker Göranson's comments that Sweden could only defend itself for a week if it were attacked.