Zelensky, Erdogan, Guterres to meet Thursday in Ukraine
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The announcement came as Russia claimed massive explosions at a military facility on the Kremlin-controlled Crimean peninsula that also damaged electrical power infrastructure were the result of "sabotage".
Russian President Vladimir Putin, meanwhile, accused the United States of trying to drag out the conflict, accusing Washington of putting Ukrainians in the firing line and supplying Kyiv with heavy weapons.
Russia's invasion of Ukraine in February has killed thousands, displaced millions from their homes and ravaged swathes of the country.
It has also blocked key grain exports, although ships have now started to set sail from the war-torn country after a landmark deal brokered by the UN and Turkey last month to relieve the global food crisis.
The UN chief will then visit the Ukrainian port city of Odessa -- one of three ports being used in the recent deal to export grain -- before heading to Turkey.
Early on Tuesday, huge fireballs erupted at a military site in Crimea where ammunition was temporarily being stored and clouds of black smoke billowed into the air, images posted on social media showed.
"As a result of an act of sabotage, a military storage facility near the village of Dzhankoi was damaged," Russian news agencies reported the defence ministry as saying.
The blasts -- caused by a fire that led ammunition to detonate -- damaged civilian infrastructure, "including power lines, a power plant, a railway track" and residential buildings, the ministry said.
The explosions come one week after at least one person was killed in similar explosions at a Russian military airbase in Crimea.
Ukraine has not directly claimed responsibility for either incident, but senior officials and the military have implied Ukrainian involvement.
Ukrainian presidential aide Mykhailo Podolyak said the explosions had likely damaged infrastructure supplying power generated at the Russian-controlled Zaporizhzhia nuclear plant to Crimea.
Kyiv and Moscow have traded accusations over a series of strikes this month on the Zaporizhzhia nuclear plant in southern Ukraine -- Europe's largest.
Zelensky warned Monday that a "catastrophe" at the Russian-controlled facility would threaten the whole of Europe.
UN spokesman Dujarric said he had "no doubt that the issue of the nuclear power plant" would be raised at Thursday's meeting.
Russia annexed Crimea from Ukraine in 2014 and has used the Black Sea region as a staging ground for its invasion.
Moscow ordered the invasion of Ukraine in February, anticipating little military resistance and hoping for a lightning takeover that would topple the government in Kyiv within hours.
But after failing to capture the capital, its forces have become entrenched in a war of attrition along a sprawling front line in the east and south.
"The situation in Ukraine shows that the US is trying to prolong this conflict," Putin said Tuesday.
Washington is "using the people of Ukraine as cannon fodder", he added.
Washington has provided key backing to Kyiv, in particular supplying long-range, precision artillery that has allowed Ukraine to strike Russian supply facilities deep inside Moscow-controlled territory.
Meanwhile, in the eastern Donbas region, which has seen most of the fighting, Ukraine said Russia had launched a "massive" offensive from an oil refinery in the recently captured city of Lysychansk in Lugansk province.
Ukraine's presidency said one woman was killed in Donetsk province, which together with Lugansk makes up the industrial Donbas region now mostly controlled by Russian forces.
Symbols of repressions
The first UN-chartered vessel departed on Tuesday from the Ukrainian port of Pivdennyi and will head to Djibouti "for delivery to Ethiopia", Ukraine's infrastructure ministry said.
The MV Brave Commander, carrying 23,000 tonnes of wheat, was able to leave after the deal last month lifted a Russian blockade of Ukraine's ports, establishing safe corridors through the naval mines laid by Kyiv.
Ukraine has said it is hoping two or three similar shipments will follow soon.
Russia's invasion has driven an economic, political and cultural wedge between Moscow and European capitals.
The prime minister of former Soviet Estonia said Tuesday her government had decided to remove all Soviet-era monuments from public spaces in the country.
"As symbols of repressions and Soviet occupation, they have become a source of increasing social tensions," Kaja Kallas wrote on Twitter.
That follows similar trends in Poland and Ukraine, which began tearing down statues of Soviet leaders in earnest after Russia-backed separatists captured swathes of the east in 2014.
Finland, meanwhile, announced plans to limit Russian tourist visas to 10 percent of current volumes beginning in September, due to rising discontent over Russian tourism amid the war in Ukraine.