A captured Russian soldier who pleaded guilty to killing a civilian was sentenced by a Ukrainian court Monday to life in prison -- the maximum -- amid signs the Kremlin may, in turn, put on trial some of the fighters who surrendered at Mariupols 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 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. He also 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 Moscows 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 mans 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 civil liberties advocate Volodymyr Yavorskyy said it was an extremely harsh sentence for one murder during the war. But Aarif Abraham, a British-based human rights lawyer, said the trial was conducted with what appears to be full and fair due process, including access to an attorney.

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 Moscows withdrawal from around Kyiv weeks ago, mass graves were discovered and streets were strewn with bodies in towns such as Bucha.

Before Shishimarins 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 OConnell, 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.

Maybe it would have happened without the Ukrainians beginning trials, OConnell said. But the timing suggests that the Ukrainians should have held back and perhaps still should, so that the Russians cant say, `Were just doing to their soldiers what they did to ours.

Russian authorities have threatened to hold trials of captured Ukrainians -- namely, fighters who held out at Mariupols 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.

Russias 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 Regiments fighters Nazis and accusing their commander without evidence of numerous atrocities. Russias top prosecutor has asked the countrys 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 Russias 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.

This is what sanctions should be: They should be maximum, so that Russia and every other potential aggressor that wants to wage a brutal war against its neighbor would clearly know the immediate consequences of their actions, said Zelenskyy, who received a standing ovation.

In other developments, nearly 50 defense leaders from around the world met Monday and agreed to send more advanced weapons to Ukraine, including missiles to protect its coast, U.S. Defense Secretary Lloyd Austin told reporters in Washington.

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.

Donetsks 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 regions 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.

On the eve of the three-month anniversary of the start of the war, Zelensky said that four missiles killed 87 people last week in the town of Desna, 55 kilometers (34 miles) north of Kyiv. The deaths were tallied after debris was cleared, he said.

The Russians have now concentrated their forces on Donbas cities and are trying to destroy all life, Zelenskyy said in his evening address to the nation.

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.

Some who fled the Donetsk region shared their suffering.

We havent been able to see the sun for three months. We are almost blind because we were in darkness for three months, said Rayisa Rybalko, who hid with her family first in their basement and then in a bomb shelter at a school before fleeing their village of Novomykhailivka. The world should have seen that.

Her son-in-law Dmytro Khaliapin said heavy artillery pounded the village. Houses are being ruined, he said. Its a horror.