KYIV, Ukraine (AP) — The Kremlin warned it may take retaliatory “military-technical” steps after Finland’s leaders said they favor joining NATO, and Sweden could do the same within days, in a historic realignment triggered by Russia's invasion of Ukraine.

Meanwhile, the first war-crimes trial of a Russian soldier since the start of the conflict was set to open Friday in Kyiv. A 21-year-old captured member of a tank unit is accused of shooting to death a civilian in the war's first week.

On the ground, Russian forces were pounding areas in central, northern and eastern Ukraine, including the last pocket of resistance in Mariupol, as part of their offensive to take the industrial Donbas region. Ukraine recaptured some towns and villages in the northeast.

Two and a half months after Russia’s invasion of Ukraine sent a shiver of fear through Moscow’s neighbors, Finland’s president and prime minister announced Thursday 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,” said Finnish President Sauli Niinisto.

Finland's Parliament still has to weigh in, but the announcement means it is all but certain to apply — and gain admission. 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 Russian President Vladimir Putin. He 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 has been critical to Kyiv's surprising ability to stymie the invasion, and the Kremlin warned anew 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.

Britain’s Defense Ministry said Friday that Russia has not made any significant advances despite concentrating forces in the Donbas after withdrawing troops from other areas.

British military officials said Russia lost “significant” elements of at least one battalion tactical group — about 1,000 troops — and equipment that were used to quickly deploy a makeshift floating bridge while trying to cross the Siverskyi Donets River west of Severodonetsk.

“Conducting river crossings in a contested environment is a highly risky maneuver and speaks to the pressure the Russian commanders are under to make progress in their operations in eastern Ukraine,” the ministry said in its daily intelligence update.

As the fighting and Russian strikes persisted, teachers were trying to restore some sense of normalcy after the war shuttered Ukraine’s schools and devastated the lives of millions of children. In Kharkiv, Ukraine’s second-largest city, lessons are being given in a subway station used as a bomb shelter that has become home for many families.

“It helps to support them mentally. Because now there is a war, and many lost their homes ... some people’s parents are fighting now,” said teacher Valeriy Leiko. In part thanks to the lessons, he said, “they feel that someone loves them.”

Primary school-age children joined Leiko around a table for history and art lessons in the subway station, where children’s drawings now line the walls.

An older student, Anna Fedoryaka, was monitoring lectures on Ukrainian literature being given by Kharkiv professor Mykhailo Spodarets online from his basement.

Internet connections were a problem, Fedoryaka said. And, “it is hard to concentrate when you have to do your homework with explosions by your window."

At least two civilians were killed on the outskirts of Kharkiv on Thursday, 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.

The Ukrainian military chief for the eastern Luhansk region said Friday that Russian forces opened fire 31 times on residential areas the day before, destroying dozens of homes, notably in Hirske and Popasnianska villages, and a bridge in Rubizhne.

Russia’s advance in the Donbas has been slow, but its forces have gained some ground and taken some villages.

In other developments, Ukrainian officials said their forces took out another Russian ship in the Black Sea, though there was no confirmation from Russia and no casualties were reported.

The Vsevolod Bobrov logistics ship was badly damaged but not thought to have sunk when it was struck while trying to deliver an anti-aircraft system to Snake Island, said Oleksiy Arestovych, an adviser to the Ukrainian president.

In April, the Ukrainian military sank the Moskva cruiser, the flagship of Russia's Black Sea fleet. In March it destroyed the landing ship Saratov.

Ukraine said Russian forces fired artillery and grenade launchers at Ukrainian troops around Zaporizhzhia, which has been a refuge for civilians fleeing Mariupol, and attacked in the Chernihiv and Sumy regions to the north.

The Ukrainian military also said Russian forces were transferring additional artillery units to border areas near Chernihiv, where overnight strikes killed at least three people. It said that Russian troops fired rockets at a school and student dormitory in Novhorod-Siversky and that some other buildings, including private homes, were also damaged.

In his evening address to the nation, Ukrainian President Volodymyr Zelenskyy condemned the assaults.

“Of course, the Russian state is in such a state that any education only gets in its way," he said. "But what can be achieved by destroying Ukrainian schools? All Russian commanders who give such orders are simply sick and incurable.”

The southern port of Mariupol has largely been reduced to smoking rubble with little food, water or medicine, or what the mayor called a “medieval ghetto." Ukrainian fighters there continued to hold out at the Azovstal steel plant, the last stronghold of resistance in the city.

Ukrainian Deputy Prime Minister Iryna Vereshchuk said negotiations were underway with Russia to win the release of 38 severely wounded Ukrainian defenders from the plant. She said Ukraine hoped to exchange them for 38 “significant” Russian prisoners of war.

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Yesica Fisch in Bakhmut, David Keyton in Kyiv, Yuras Karmanau in Lviv, Mstyslav Chernov in Kharkiv, Jari Tanner in Helsinki, and other AP staffers around the world contributed to this report.