KYIV, Ukraine (AP) -- Finland's leaders Thursday came out in favor of applying to join NATO, and Sweden could do the same within days, in a historic realignment on the continent 2½ months after Russian President Vladimir Putin's invasion of Ukraine sent a shiver of fear through Moscow's neighbors.
The Kremlin reacted by warning it will be forced to take retaliatory "military-technical" steps.
On the ground, meanwhile, Russian forces pounded areas in central, northern and eastern Ukraine, including the last pocket of resistance in Mariupol, as part its offensive to take the industrial Donbas region, while Ukraine recaptured some towns and villages in the northeast.
The first war-crimes trial of a Russian soldier since the start of the conflict is set to open today in Kyiv. A 21-year-old captured member of a tank unit is accused of shooting to death a civilian on a bicycle during the opening week of the war.
Finland's president and prime minister announced that the Nordic country should apply right away for membership in NATO, the military defense pact founded in part to counter the Soviet Union.
"You (Russia) caused this. Look in the mirror," Finnish President Sauli Niinisto said this week.
While the country's Parliament still has to weigh in, the announcement means Finland is all but certain to apply -- and gain admission -- though the process could take months to complete. Sweden, likewise, is considering putting itself under NATO's protection.
That would represent a major change in Europe's security landscape: Sweden has avoided military alliances for more than 200 years, while Finland adopted neutrality after its defeat by the Soviets in World War II.
Public opinion in both nations shifted dramatically in favor of NATO membership after the invasion, which stirred fears in countries along Russia's flank that they could be next.
Such an expansion of the alliance would leave Russia surrounded by NATO countries in the Baltic Sea and the Arctic and would amount to a stinging setback for Putin, who had hoped to divide and roll back NATO in Europe but is instead seeing the opposite happen.
NATO Secretary-General Jens Stoltenberg has said the alliance would welcome Finland and Sweden with open arms.
Russia's Foreign Ministry warned that Moscow "will be forced to take retaliatory steps of military-technical and other characteristics in order to counter the emerging threats to its national security."
NATO's funneling of weapons and other military support to Ukraine already has been critical to Kyiv's surprising success in stymieing the invasion, and the Kremlin warned anew in chilling terms Thursday that the aid could lead to direct conflict between NATO and Russia.
"There is always a risk of such conflict turning into a full-scale nuclear war, a scenario that will be catastrophic for all," said Dmitry Medvedev, deputy head of Russia's Security Council.
While Russia's advance in the Donbas has been slow, its forces have gained some ground and taken some villages.
Four civilians were killed Thursday in three communities in the Donetsk region, which is part of the Donbas, the regional governor reported.
Britain's Defense Ministry said Russia's focus on the Donbas has left its remaining troops around the northeastern city of Kharkiv vulnerable to counterattack from Ukrainian forces, which recaptured several towns and villages around the city.
Russian strikes Thursday killed at least two civilians on the outskirts of Kharkiv, Ukraine's second-largest city, local authorities said.
The attacks also damaged a building housing a humanitarian aid unit, municipal offices and hospital facilities, Vyacheslav Zadorenko, the mayor of the suburban town of Derhachi, wrote in a Telegram post.
None of the sites "had anything to do with military infrastructure," Zadorenko said.
Local residents fil bottles with drinking water brought by Donetsk People Republic Emergency Situations Ministry in Mariupol, in territory under the government of the Donetsk People's Republic, eastern Ukraine, Thursday, May 12, 2022. (AP Photo)
British Prime Minister Boris Johnson, left, and Finland's President Sauli Niinisto shake hands after signing a security assurance, at the Presidential Palace in Helsinki, Finland, Wednesday, May 11, 2022. Britain has signed a security assurance with Sweden and its neighbor Finland, that are both pondering whether to join NATO following Russia's invasion of Ukraine, pledging to "bolster military ties" in the event of a crisis and support both countries should they come under attack. (AP Photo/Frank Augstein, Pool)
An elderly woman walks inside a metro station being used as a bomb shelter in Kharkiv, Ukraine, Thursday, May 12, 2022. (AP Photo/Mstyslav Chernov)
Associate professor of Ukrainian literature Mykhailo Spodarets gives an online lesson from the basement of his house, used as a temporary shelter, in Kharkiv, Ukraine, Thursday, May 12, 2022. (AP Photo/Mstyslav Chernov)
A man shows a part of a missile found in a residential area after Russian shelling in Bakhmut, Donetsk region, Ukraine, Thursday, May 12, 2022. (AP Photo/Andriy Andriyenko)
A baby bed is seen inside an apartment building damaged by Russian shelling in Bakhmut, Donetsk region, Ukraine, Thursday, May 12, 2022. (AP Photo/Andriy Andriyenko)
A early 200-year old monument to legendary Prince Volodymyr, a Kyiv landmark and one of the symbols of the Ukrainian capital, is wrapped to protect it from the Russian shelling in Kyiv, Ukraine, Thursday, May 12, 2022. (AP Photo/Efrem Lukatsky)
A early 200-year old monument to legendary Prince Volodymyr, a Kyiv landmark and one of the symbols of the Ukrainian capital, is wrapped to protect it from Russian shelling in Kyiv, Ukraine, Thursday, May 12, 2022. (AP Photo/Efrem Lukatsky)
A crater of an explosion after Russian shelling is seen next to a damaged apartment building in Bakhmut, Donetsk region, Ukraine, Thursday, May 12, 2022. (AP Photo/Andriy Andriyenko)