Alarmed by Russian aggression in Eastern Europe, Finland and Sweden announced a new military cooperation agreement on Thursday that could see the two Scandinavian countries go to war together in the event of an attack.

Although the arrangement would seem to mimic the mutual defense charter of NATO, to which neither Finland nor Sweden belongs, Stockholm’s defense minister, Peter Hultqvist, said the cooperation was not a formal alliance.

“By planning for various crisis scenarios, we create preparations to use them in a given situation,” he told the Dagens Nyheter newspaper.

Whether or not we end up implementing these proposals is a decision that has to be made at government level in that situation and then confirmed by the parliaments in the two countries.

Hultqvist’s Finnish counterpart, Carl Haglund, similarly told the TT news agency, “This gives us a concrete ability to work together, first and foremost in peacetime but also in times of crisis should we choose to.”

New forms of cooperation may include increased communication and shared military bases.

Late last year, Finland and Sweden agreed with other Northern European countries, including neighboring Denmark and Norway, which are both members of NATO, to improve intelligence sharing and joint air force training in the face of renewed Russian threats.

Russia has played a cat-and-mouse game with its western neighbors since it occupied and annexed the Crimean Peninsula from Ukraine last year. The incident, which came after a row with the European Union over the bloc’s improving relations with the former Soviet republic and the overthrow of a relatively pro-Russian president in Kiev, alarmed especially Eastern European nations about Russian revanchism.

Fighter jets from Scandinavian and NATO countries have regularly intercepted Russian strategic bombers and planes approaching their airspace through last year as tensions over the standoff in Ukraine mounted.

In October, Sweden scrambled its naval forces in search of a suspected Russian submarine in its waters.

Support for joining NATO surged in the wake of the incident. A Novus poll conducted for TV4 showed 37 percent of the traditionally neutral Swedes in favor of NATO membership against 36 percent who opposed joining the alliance. Earlier in the year, only 28 percent had been in favor against 56 percent opposed.

A majority of Fins still opposes NATO membership. Prime Minister Alexander Stubb is not among them. “We have to aim at maximising Finland’s national security and being part of decisionmaking and that happens best as a NATO member,” he told the Reuters news agency shortly before taking over as premier from Jyrki Katainen in June.