Moscow said that Finnish accession, which would add hundreds of miles to NATO’s shared border with Russia, would threaten its security. Kremlin spokesman Dmitry Peskov said Finnish membership could require new measures by Russia to “balance the situation.”

Finnish President Sauli Niinisto and Prime Minister Sanna Marin, announcing their positions after weeks of internal deliberations, said the militarily nonaligned nation must “apply for NATO membership without delay.”

“As a member of NATO, Finland would strengthen the entire defence alliance,” they said in a statement. The decision, which must be approved by Finland’s Parliament, is expected to be finalized in coming days.

Finnish Foreign Minister Pekka Haavisto said President Vladimir Putin’s assault on Ukraine illustrates the Russian leader’s willingness to send troops into neighboring nations and has dramatically altered the security outlook — not only for Finland. “The war started by Russia jeopardizes the security and stability of the whole of Europe,” he told European lawmakers.

Inside Mariupol’s besieged steel plant, a symbol of courage and terror

It was not immediately clear what steps NATO nations might take to protect Finland and Sweden from any retaliation by Russia until they are formally brought under NATO’s mutual defense umbrella. That process, Western officials have said, could be completed by the time leaders of the alliance gather in Spain in late June.

British Prime Minister Boris Johnson on Wednesday announced new measures to bolster Finland and Sweden’s security, including greater intelligence sharing and joint training.

In a sign of increasing global concern about the war, the United Nations voted Thursday to deepen an investigation of alleged human rights violations during Russia’s offensive, now focused on Ukraine’s east and south after a failed attempt to capture the capital, Kyiv.

The vote by members of the U.N. Human Rights Council in Geneva was aimed at focusing an existing probe on events around Kyiv and other areas that were occupied by Russian forces early in the conflict. In many places, the withdrawal of Russian troops was followed by the discovery of widespread civilian deaths, including cases of apparent execution and torture.

U.N. High Commissioner for Human Rights Michelle Bachelet said that more than 1,000 civilian bodies had been recovered around Kyiv and that U.N. officials are collecting evidence about incidents that “may amount to war crimes.”

“These killings of civilians often appeared to be intentional, carried out by snipers and soldiers,” she said in a video message.

The war has prompted more than 6 million people to flee Ukraine since late February, creating a new refugee crisis as waves of Ukrainians, mostly women and children, pour westward into Europe and into other areas. About 1.6 million Ukrainians have returned as conditions have stabilized around Kyiv and in the west, new data from the U.N. refugee agency shows.

Moscow is seeking to solidify control of eastern areas where Russian-backed separatists have staged a prolonged fight against Kyiv. The Kremlin also wants to control coastal areas that would create a land bridge from Russian territory to the Crimean Peninsula, which Moscow annexed in 2014.

The coastal city of Mariupol has been particularly pulverized by Russian bombardment. A focal point of that fight has been around the massive Azovstal steel plant, where civilians and Ukrainian fighters holed up amid heavy shelling.

Deputy Prime Minister Iryna Vereshchuk said that “very difficult negotiations” were ongoing with Russia to permit the evacuation of 38 wounded Ukrainian fighters from the plant as part of a potential prisoner swap.

“We work step by step,” Vereshchuk wrote on Telegram.

NATO nations have increased the flow of weapons and other aid to leaders in Kyiv and imposed punishing sanctions on Russia, bringing Moscow’s relations with the West to their worst point since the Cold War.

Underscoring the growing hostility, European Commission President Ursula von der Leyen on Thursday described Russia as “the most direct threat” to the global order.

Von der Leyen, speaking during a visit to Japan, cited Russia’s invasion of Ukraine and its ties with China as proof of the peril Moscow poses to Europe and beyond. Western officials have said that a weak response to Russia’s invasion would validate Putin’s use of force to expand Russia’s borders and potentially embolden autocrats worldwide.

Putin has long cited NATO’s eastward expansion — from its founding group of 12 nations in 1949, all from Western Europe and North America, to its 30 members today, including a clutch of former Soviet and Warsaw Pact states — as a major threat to Russian security.

“The expansion of NATO does not make our continent more stable and secure,” Peskov told journalists Thursday, according to the Russian news outlet Interfax. “NATO is moving in our direction,” he said.

Those beliefs could be particularly sharp when it comes to Finland and Sweden, which security experts say punch above their weight in military terms. The two countries have long worked closely with NATO nations, conducting joint training and sending troops to the NATO-led mission in Afghanistan. Just before Putin’s offensive began, Finland finalized its purchase of 64 F-35 stealth fighter jets.

How Putin’s brutal war in Ukraine pushed Finland toward NATO

Russia’s Foreign Ministry characterized Finland’s decision to seek NATO membership as “a radical change” in the nation’s foreign policy, saying it contravened a nonalignment stance that — according to the ministry — has served Moscow and Helsinki well.

Dmitry Medvedev, the deputy chairman of Russia’s Security Council and a former Russian president, said that NATO’s support of Ukraine, along with military exercises in countries bordering Russia, “increase the likelihood of a direct and open conflict.”

“This kind of conflict is always at risk of turning into a full-fledged nuclear war,” Medvedev said.

The Foreign Ministry said Russia would be “forced to take retaliatory steps, both of a military-technical and other nature.”

Russia’s invasion has also driven countries in the former Soviet sphere closer to the West. Ukraine and Moldova are now actively seeking membership in the European Union. Neither country is a NATO member. The prospect of Ukrainian membership has been characterized as a red line for Moscow.

The Finnish leaders’ statement was greeted with expressions of support across Europe and promises to keep the application process as short as possible. NATO Secretary General Jens Stoltenberg told Reuters that “Finland is one of NATO’s closest partners, a mature democracy, a member of the European Union and an important contributor to Euro-Atlantic security.”

Swedish Foreign Affairs Minister Ann Linde on Thursday said her country should take the Finnish assessments “into account” when making its own decision on NATO membership. The Swedish tabloid Expressen, citing unnamed sources, reported that a decision from Sweden on joining the defense alliance could come as soon as Monday.

Timsit and Parker reported from London. Cheng reported from Seoul. Ryan reported from Washington. Andrew Jeong in Seoul, Michelle Ye Hee Lee in Tokyo, Kim Bellware in Chicago, and Marisa Iati and Jaclyn Peiser in Washington contributed to this report.