Russia has slammed Finland’s plans to apply to join NATO imminently, claiming it would “be forced” to retaliate if the long-neutral country joined the military alliance.

“Finland joining NATO is a radical change in the country’s foreign policy,” the Russian Foreign Ministry said in a statement Thursday. “Russia will be forced to take retaliatory steps, both of a military-technical and other nature, in order to stop threats to its national security arising.”

The statement comes shortly after Finland’s president, Sauli Niinisto, and Prime Minister Sanna Marin said the country should apply to join NATO “without delay.”

It’s is the strongest sign yet that Finland will make a formal application to join NATO. Membership would be historic for the Nordic country, which has had a decadeslong policy of military neutrality.