Margot Wallström | EPA/JULIEN WARNAND Swedish-Russian relations enter deep freeze Stockholm rejects an apparent attempt to influence foreign and security policy.

Relations between Sweden and Russia are “chillier than ever,” Swedish Foreign Minister Margot Wallström said Thursday, after Russia indicated it would take "retaliatory measures" if Sweden joined NATO.

Wallström told the Expressen tabloid she had met with Russian Ambassador Viktor Tatarintsev to seek an explanation after a Russian foreign ministry spokeswoman said last week that “Swedish membership in NATO would have politico-military and foreign policy consequences, and would require retaliatory measures from Russia.”

Wallström said threats from Russia were "unacceptable."

"We also wanted to establish that Sweden decides its own foreign and security policies and that we do not accept attempts to, through threats, influence us in this or other matters.”

In October last year, Sweden unsuccessfully searched for Russian submarines after suspected sightings near Stockholm. There have also been reports of Russian fighter pilots behaving aggressively near Swedish airspace, including firing flares and flying dangerously close to Swedish planes.

The perceived threat from Russia has reinvigorated the decades-old debate in Sweden about whether it should join the Western military alliance in Europe. As POLITICO reported last month, Stockholm has named a special ambassador to evaluate the relationship with NATO and other international organizations. The Finnish government has also formed a committee to study NATO membership.

An opinion poll on Monday found 41 percent of Swedes think the country should join NATO while 39 percent are opposed.

Also on Monday, Sweden signed a deal on military co-operation with Poland — which is a member of NATO.

Swedish Defense Minister Peter Hultqvist said Sweden had decided reinforce its cooperation with NATO and to boost defense spending by 11 percent over five years, due to more Russian warships and warplanes in the Baltic Sea.