Ukraine has accused the Kremlin of "food terrorism" after Vladimir Putin's troops stole tonnes of grain from Russian-controlled areas.
Kyiv said Russian soldiers are targeting farmers' harvests in the south and the eastern region of Donbas, where fierce fighting continues.
Government officials said around 400,000 tonnes of grain had been looted from occupied Luhansk, Donetsk, Kherson and Zaporizhzhia.
"Chechen soldiers, fighting for Russia, act like criminals in the 1990s," Ivan Fedorov, mayor of Melitopol, an occupied city in the Zaporizhzhia region, told CNN.
"First they offer to buy grain for a ridiculously low price," he added. "But if you don't agree, they take everything from you for nothing."
Officials have alleged that Russia is deliberately trying to cause a famine in Ukraine, drawing comparisons to the Holodomor famine engineered by Soviet dictator Joseph Stalin.
Around 4 million people in Ukraine died as a result of the crisis, most of them farmers and rural residents.
On Thursday, Ukrainian fighters in the tunnels underneath Mariupol's pulverized steel plant held out in an increasingly desperate and perhaps doomed effort to deny Moscow what would be its biggest success of the war yet: the complete capture of the strategic port city.
The bloody battle came amid growing speculation that Russian president Putin wants to present the Russian people with a battlefield triumph — or announce an escalation of the war — in time for Victory Day on Monday.
Victory Day is the biggest patriotic holiday on the Russian calendar, marking the Soviet Union's triumph over Nazi Germany.
Some 2,000 Ukrainian fighters, by Russia's most recent estimate, were holed up at Mariupol's sprawling Azovstal steelworks, the last pocket of resistance in a city largely reduced to rubble over the past two months.
A few hundred civilians were also believed trapped there.
The defenders will "stand till the end. They only hope for a miracle," Kateryna Prokopenko said after speaking by phone to her husband, a leader of the steel plant defenders. "They won't surrender."
She said her husband, Azov Regiment commander Denys Prokopenko, told her he would love her forever.
"I am going mad from this. It seemed like words of goodbye," she said.
Ukraine’s president Volodymyr Zelensky said the attack was preventing the evacuation of civilians remaining in the plant's underground bunkers.
"Just imagine this hell! And there are children there," he said late Thursday in his nightly video address. "More than two months of constant shelling, bombing, constant death."
The Russians managed to get inside with the help of an electrician who knew the layout, said Anton Gerashchenko, an adviser to Ukraine's Internal Affairs Ministry.
"He showed them the underground tunnels which are leading to the factory," Gerashchenko said in a video posted late Wednesday. "Yesterday, the Russians started storming these tunnels, using the information they received from the betrayer."
The Kremlin denied its troops were storming the plant.
The fall of Mariupol would deprive Ukraine of a vital port, allow Russia to establish a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014, and free up troops to fight elsewhere in the Donbas, the eastern industrial region that the Kremlin says is now its chief objective.
Capt Sviatoslav Palamar, deputy commander of the Azov Regiment, pleaded on Ukrainian TV for the evacuation of civilians and wounded fighters from the steelworks, saying soldiers were "dying in agony due to the lack of proper treatment".
The Kremlin’s demands that troops surrender have been refused. Russia has also accused them of preventing the civilians from leaving.
The head of the United Nations said another attempt to evacuate civilians from Mariupol and the plant was underway. UN secretary-general, Antonio Guterres, said: "We must continue to do all we can to get people out of these hellscapes."
More than 100 civilians were rescued from the steelworks over the weekend. But many previous attempts to open safe corridors from Mariupol have fallen through, with Ukraine blaming shelling and firing by the Russians.
Meanwhile, 10 weeks into the devastating war, Ukraine's military claimed it had recaptured some areas in the south and repelled other attacks in the east, further frustrating Putin's ambitions after his abortive attempt to seize Kyiv.
