KYIV (AP) -- A Ukrainian court sentenced a captured Russian soldier to the maximum penalty of life in prison for killing a civilian and the Kremlin hinted that it may put on trial some of the fighters who surrendered at Mariupol's steelworks.
Meanwhile, in a rare public expression of opposition to the war from the ranks of the Russian elite, a veteran Kremlin diplomat resigned and sent a scathing letter Monday to foreign colleagues in which he said of the invasion, "Never have I been so ashamed of my country as on Feb. 24."
Also, Ukrainian President Volodymyr Zelenskyy called for "maximum" sanctions against Russia in a video address to world leaders and executives at the World Economic Forum in Davos, Switzerland. Separately, he revealed one of the deadliest single strikes of the war, a missile attack on a village near Kyiv that killed almost 90 people.
And on the battlefield, heavy fighting raged in the Donbas in the east, where Moscow's forces have stepped up their bombardment. Cities not under Russian control were constantly shelled, and one Ukrainian official said Russian forces targeted civilians trying to flee.
In the first of what could be a multitude of war crimes trials held by Ukraine, Russian Sgt. Vadim Shishimarin, 21, was sentenced for the killing of a 62-year-old man who was shot in the head in a village in the northeastern Sumy region in the opening days of the war.
Shishimarin, a member of a tank unit, had claimed he was following orders, and he apologized to the man's widow in court.
His Ukraine-appointed defense attorney, Victor Ovsyanikov, argued his client had been unprepared for the "violent military confrontation" and mass casualties that Russian troops encountered when they invaded. He said he would appeal.
Ukrainian prosecutors are investigating thousands of potential war crimes. Russian forces in Mariupol bombed a theater where civilians were sheltering and struck a maternity hospital. In the wake of Moscow's withdrawal from around Kyiv weeks ago, mass graves were discovered and streets were strewn with bodies in towns such as Bucha.
Before Shishimarin's sentencing, Kremlin spokesman Dmitry Peskov said that Moscow was unable to defend the soldier but will consider trying to do so "through other channels."
Mary Ellen O'Connell, an expert on international law at the University of Notre Dame, said that putting Shishimarin on trial could prove "extremely detrimental to Ukrainian soldiers in the hands of Russia." She said Russia may decide to hold "show trials" of Ukrainians to boost the morale of its own soldiers and spread disinformation.
Russian authorities have threatened to hold trials of captured Ukrainians -- namely, fighters who held out at Mariupol's shattered steel plant, the last stronghold of resistance in the strategic southern port city. They surrendered and were taken prisoner last week, at which point Moscow claimed the capture of Mariupol was complete.
Russia's main investigative body said it intends to interrogate the Mariupol defenders to "identify the nationalists" and determine whether they were involved in crimes against civilians.
Russian authorities have seized upon the far-right origins of one of the regiments there, calling the Azov Regiment's fighters "Nazis" and accusing their commander without evidence of "numerous atrocities." Russia's top prosecutor has asked the country's Supreme Court to designate the Azov Regiment a terrorist organization.
Family members of the fighters have pleaded for their eventual return to Ukraine as part of a prisoner swap.
Elsewhere, Boris Bondarev, a veteran Russian diplomat at the U.N. office at Geneva, quit and sent a letter denouncing the "aggressive war unleashed" by Russian President Vladimir Putin. Bondarev told The Associated Press: "It is intolerable what my government is doing now."
In his letter, Bondarev said those who conceived the war "want only one thing -- to remain in power forever, live in pompous tasteless palaces, sail on yachts comparable in tonnage and cost to the entire Russian Navy, enjoying unlimited power and complete impunity."
He also said Russia's Ministry of Foreign Affairs is all about "warmongering, lies and hatred."
At the Davos forum, Zelenskyy said sanctions against the Kremlin must go further. He urged an embargo on Russian oil, a complete cutoff of trade and a withdrawal of foreign companies from the country.
Later, in his evening address to the nation, Zelenskyy said that four missiles killed 87 people last week in the town of Desna, 55 km north of Kyiv. The deaths were tallied after debris was cleared, he said.
On the battlefield, Russian forces increased their bombardment of the Donbas, the eastern industrial heartland of coal mines and factories that Russia is bent on capturing.
Donetsk's regional governor, Pavlo Kyrylenko, said three civilians died in Russian attacks there Monday and heavy fighting continued near the Luhansk region. The Donbas consists of the Donetsk and Luhansk regions.
He said the Russians were decimating cities in their attempt to take them over. Only about 320,000 people out of the region's prewar population of 1.6 million remain, and Russian forces are targeting evacuation efforts, he said.
"They are killing us. They are killing the locals during evacuation," Kyrylenko said.
In the Luhansk region, U.N. spokesman Stephane Dujarric said, local authorities reported that a bridge leading to the administrative center of Sievierodonetsk was destroyed, leaving the partially encircled city reachable by just one road.