HELSINKI—Sweden and Finland will further strengthen their security cooperation in light of Russia’s invasion of Ukraine, the prime ministers of both countries said on Saturday, but they were non-committal on joining NATO. “Russia’s war against a sovereign European nation puts the European security order at risk. In this changing security environment, Finland and Sweden will further strengthen our cooperation,” Finnish Prime Minister Sanna Marin told reporters in Helsinki at a joint news conference with Swedish Prime Minister Magdalena Andersson. Sweden and Finland have strong historic ties and a close military cooperation, including joint drills and information sharing. The invasion, called a “special military operation” by Russia, has also forced a quick change in attitude towards NATO in both countries, which are currently outside the NATO-alliance. Polls in recent days showed a majority for joining NATO in both countries for the first time ever. Marin said it was “understandable” that more …