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(Adds death toll and battle report from Ukrainian side, U.S. not extending waiver on Russian payments to U.S. bondholders)
* Russia tries to encircle Ukrainian troops in twin cities
* Kharkiv reopens metro after Russian forces driven back
* Corpses still being found in Russian-held Mariupol
* EU's von der Leyen says Moscow weaponizing food
By Pavel Polityuk and Conor Humphries
KYIV/SLOVYANSK, Ukraine, May 24 (Reuters) - Russian forces waged an all-out assault on Tuesday to encircle Ukrainian troops in twin eastern cities straddling a river, a battle that could determine the success or failure of Moscow's main campaign in the industrial heartland of Donbas.
Russia is attempting to seize the separatist-claimed Donbas' two provinces, Donetsk and Luhansk, and trap Ukrainian forces in a pocket on the main eastern front.
Russian forces took control of three towns in the Donetsk region including Svitlodarsk, regional governor Pavlo Kyrylenko told a local affiliate of Radio Free Europe/Radio Liberty.
"The situation on the (eastern) front is extremely difficult because the fate of this country is perhaps being decided (there) right now," said Ukrainian Defence Ministry spokesman Oleksandr Motuzyanyk.
The easternmost part of the Ukrainian-held Donbas pocket, the city of Sievierodonetsk on the east bank of the Siverskiy Donets River and its twin Lysychansk on the west bank, have become the pivotal battlefield there. Russian forces were advancing from three directions to encircle them.
"The enemy has focused its efforts on carrying out an offensive in order to encircle Lysychansk and Sievierodonetsk," said Serhiy Gaidai, governor of Luhansk province, where the two cities are among the last territory still held by Ukraine.
Ukraine's military said it had repelled nine Russian attacks on Tuesday in the Donbas where Moscow's troops had killed at least 14 civilians, using aircraft, rocket launchers, artillery, tanks, mortars and missiles.
Reuters could not immediately verify the information.
In a sign of Ukrainian success elsewhere, authorities in its second-largest city Kharkiv re-opened the underground metro, where thousands of civilians had sheltered for months under relentless bombardment.
The move came after Ukraine pushed Russian forces largely out of artillery range of the northern city, as they did from the capital Kyiv in March.
WORLD WAR THREE?
Three months into the invasion, Moscow still has only limited gains to show for its worst military losses in decades, while much of Ukraine has suffered devastation in the biggest attack on a European state since 1945.
More than 6.5 million people have fled abroad, uncounted thousands have been killed and cities have been reduced to rubble.
The war has also caused growing food shortages and soaring prices due to sanctions and disrupted supply chains. Both Ukraine and Russia are major exporters of grain and other commodities.
European Commission chief Ursula von der Leyen accused Russia of using food supplies as a weapon.
"In Russian-occupied Ukraine, the Kremlin's army is confiscating grain stocks and machinery (...) And Russian warships in the Black Sea are blockading Ukrainian ships full of wheat and sunflower seeds," she told the World Economic Forum in Davos, Switzerland.
Billionaire financier George Soros, also speaking in Davos, said Russia's invasion of Ukraine may have marked the start of World War Three.
"The best and perhaps only way to preserve our civilization is to defeat Putin as soon as possible," he said.
Underlining the global tensions unleashed by the war, Japan - a key U.S. ally in Asia - scrambled jets on Tuesday after Russian and Chinese warplanes neared its airspace during a visit to Tokyo by U.S. President Joe Biden.
Meanwhile, in a move that could push Moscow closer to the brink of default, the Biden administration announced it would not extend a waiver set to expire on Wednesday that enabled Russia to pay U.S. bondholders.
Moscow had been allowed to keep paying interest and principal and avert default on its government debt.
Comments by senior Russian officials on Tuesday also suggested plans for a drawn-out conflict ahead.
Defence Minister Sergei Shoigu said Russia was deliberately advancing slowly to avoid civilian casualties. Nikolai Patrushev, head of Putin's security council, said Moscow would fight as long as necessary to eradicate "Nazism" in Ukraine, a justification for the war that the West calls baseless.
In Kharkiv, hundreds of people were still living underground in trains and stations when the authorities asked them to make way on Tuesday.
"Everyone is crazily scared, because there is still shelling, the rocket attacks haven't been stopped," said Nataliia Lopanska, who had lived in a metro train for nearly the entire duration of the war.
The Donbas fighting follows Russia's biggest victory in months: the surrender last week of Ukraine's garrison in the port of Mariupol after a siege in which Kyiv believes tens of thousands of civilians died.
Petro Andryushchenko, an aide to Mariupol's Ukrainian mayor now operating outside the Russian-held city, said the dead were still being found in the rubble.
Around 200 decomposing bodies were buried in debris in a basement of one high-rise building, he said. Locals had refused to collect them and Russian authorities had abandoned the site, leaving a stench across the district.
Highlighting the obstacles to a diplomatic resolution of the conflict, a new poll on Tuesday showed 82% of Ukrainians believe their country should not sign away any territory as part of a peace deal with Russia.
In Russia, where criticism of what it calls a "special operation" is banned and independent media has been shut, jailed opposition leader Alexei Navalny used a court appearance by video link from a prison colony to denounce the "stupid war which your Putin started".
"One madman has got his claws into Ukraine and I do not know what he wants to do with it - this crazy thief," he said.
(Reporting by Oleksandr Kozhukhar in Lviv, Pavel Polityuk and Natalia Zinets in Kyiv, Vitaliy Hnidiy in Kharkiv and Reuters journalists in Mariupol and Slovyansk; Writing by Peter Graff, Gareth Jones and Costas Pitas; Editing by Nick Macfie, Jon Boyle, Tomasz Janowski and Cynthia Osterman)