US Secretary of State Antony Blinken has accused Moscow of forcibly deporting up to 1.6 million Ukrainians to Russia.
Mr Blinken said Moscow was partaking in a deliberate criminal operation to depopulate parts of Ukraine.
In a statement a day before the Ukraine Accountability Conference in the Hague on alleged war crimes in Ukraine, Mr Blinken said the Kremlin is conducting a "filtration" operation to relocate Ukrainians from the occupied east and south to areas deep inside Russia.
"The unlawful transfer and deportation of protected persons is a grave breach of the Fourth Geneva Convention on the protection of civilians and is a war crime," Mr Blinken said.
He added that estimates from sources - including the Russian government itself - indicate that from 900,000 to 1.6 million Ukrainian citizens have been taken from their homes into Russia, including to isolated areas in the Russian Far East.
The number includes some 260,000 children, some of whom are being deliberately separated from parents to be put up for adoption in Russia, Mr Blinken said.
He said the filtration program appears to have been planned early and matches similar operations that Russia undertook in other wars, including in Chechnya.
Mr Blinken said it is "imperative" to hold the Russians accountable.
"This is why we are supporting Ukrainian and international authorities' efforts to collect, document, and preserve evidence of atrocities," he said.
Mr Blinken's comments come after at least five people have been killed in Russian shelling in the region surrounding the embattled Ukrainian port city of Mykolaiv near the Black Sea.
The presidential deputy chief of staff Kyrylo Tymoshenko said a barrage of 28 rocket strikes had damaged a hospital and homes in several parts of the Mykolaiv region.
"There were also artillery strikes in the Vitovsk district and according to preliminary information five civilians were killed," he said.
The Mykolaiv region governor Vitaly Kim said earlier today that several villages were shelled during the night without casualties.
Russia's defence ministry said in a briefing it had targeted Mykolaiv with "high-precision surface-to-air missiles," claiming to have killed dozens of soldiers east of the city.
A coastal city with an estimated pre-war population of around 475,000 residents, Mykolaiv lies between Ukraine's largest port of Odessa and Kherson, a city seized by Russian troops early into the February invasion.
Ukrainian military said it had launched long-range rocket attacks on Russian forces in the south and destroyed an ammunition store.
The strike on Nova Kakhovka in the Kherson region killed 52 people, Ukraine's military has said. The town's Russia-installed authorities said that at least seven people had been killed and around 70 injured, Russia's TASS news agency reported.
The strike came after the US supplied Ukraine with advanced HIMARS mobile artillery systems which Kyiv says its forces are using with growing efficiency.
Reuters could not independently verify the battlefield accounts.
"Based on the results of our rocket and artillery units, the enemy lost 52 (people), an Msta-B howitzer, a mortar and seven armoured and other vehicles, as well as an ammunition depot in Nova Kakhovka," Ukraine's southern military command said in a statement.
Pro-Russian officials said the strike killed civilians.
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The area is of strategic importance because of its Black Sea access, once thriving agricultural industry and location just north of Russian-annexed Crimea.
Unverified videos posted on social media showed an immense fireball erupting into the night sky. Images released by Russian state media showed a wasteland covered in rubble and the remains of buildings.
An official from the Russian-backed local administration said that Ukraine had used the HIMARS missiles and that they had destroyed warehouses containing salt petre, a chemical compound which can be used to make fertiliser or gunpowder.
"There are still many people under the rubble. The injured are being taken to the hospital, but many people are blocked in their apartments and houses," Vladimir Leontyev, head of the Russia-installed Kakhovka District military-civilian administration, was quoted by TASS as saying.
He said that warehouses, shops, a pharmacy, petrol stations and a church had been hit.
Russia continued to pound eastern Ukraine in an effort to gain control of Donetsk province and the entire industrial Donbas region. Moscow earlier this month captured Luhansk province, which makes up the rest of the Donbas.
Russia says it wants to wrest the Donbas from Ukraine on behalf of Moscow-backed separatists in two self-proclaimed people's republics whose independence it recognised on the eve of the war.
Ukraine is bracing for what it expects will be a massive new Russian offensive in the east. Regional Donetsk Governor Pavlo Kyrylenko said there was a significant buildup of Russian troops, particularly in the Bakhmut and Siversky areas, and around Sloviansk and Kramatorsk.
The entire front line in the region was under constant shelling as Russian troops tried to break through but they were being repelled, he said.
Further east in Donbas, Ukrainian forces launched a "massive air strike" on an air defence unit in Luhansk, pro-Russian militia officer Andrey Marochko said in his Telegram channel, according to TASS news agency.
Russia's campaign in Ukraine, which it calls a "special military operation," is nearing five months old and is Europe's biggest conflict since WWII.
Russia says it sent troops into Ukraine on 24 February to demilitarise the country and rid it of nationalists threatening Russian speakers there. Ukraine and Western countries say Russia's claims are a baseless pretext to attack.
The conflict has laid waste to Ukrainian cities and caused 5.2 million people to flee the country, according to the United Nations.
The UN human rights office said today that 5,024 civilians had been killed in Ukraine since the invasion began, adding that the real toll was likely much higher.
The conflict has blocked exports of Ukraine's grain, exacerbating a global food crisis. More than 20 million tonnesof grain are stuck in silos at the key Black Sea port of Odesa.
Turkish Defence Minister Hulusi Akar said military delegations from Ukraine, Russia and Turkey would meet UN officials in Istanbul today to discuss a possible deal to resume safe exports of Ukrainian grain.
Ukraine is a vital exporter of wheat and grains, such as barley and maize. It has also supplied nearly half of all the sunflower oil traded on global markets.
UN Secretary-General Antonio Guterres tried to play down expectations from the Istanbul talks.
"We are working hard indeed, but there is still a way to go," the UN chief told reporters.
As Russia blockades Ukraine's main Black Sea ports, Ukraine Deputy Infrastructure Minister Yuriy Vaskov said grain shipments via the Danube River had increased with the reopening of the Bystre canal, which provides access to small inland river ports.
Ukraine expects monthly grain exports to rise by 500,000 tonnes as a result, Mr Vaskov said.Ukraine is also negotiating with Romania and the European Commission about increasing shipments through the Sulina canal, he said.
