June 20 (UPI) -- Russian forces are prepared to launch a large-scale offensive in Ukraine, Ukrainian officials warned Monday.
Oleksiy Arestovych, adviser to Ukrainian President Volodymyr Zelensky, warned that Russia had amassed missiles in the Black Sea in preparation for a launch.
"All six carriers of Russian cruise missiles lined up in the Black Sea and are most likely preparing for a massive launch of missiles," Arestovych warned on social media.
Zelensky had cautioned in his nightly address Sunday that Russia was likely preparing an offensive this week as the European Union is set to debate whether to make Ukraine an official candidate to join the bloc.
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Russia also captured a key suburb of Severodonetsk on Monday and appeared to make another run at Ukraine's second-largest city, Kharkiv, with a new round of shelling, officials said.
Luhansk regional military leader Serhiy Haidai said Moscow seized the suburb of Metelkine, located just east of Severodonetsk, as Russian forces moved closer to controlling the key business hub in the Donbas region in a battle that Ukraine's forces have contested for weeks.
Analysts say Severodonetsk is the largest city in eastern Ukraine that's not yet under total Russian control.
Haidai said Russian troops have increased their attacks on Ukrainian positions in and around Severodonetsk via shelling and airstrikes.
"[Russia's military is] working hard on the Severodonetsk industrial zone and the outskirts of the city," Haidai said, according to CNN. "The same is true in the Toshkivka and Ustynivka districts.
"[They] want to make a breakthrough there, and for this purpose, they have gathered a large amount of equipment there. Fighting is taking place in many villages around Severodonetsk and Lysychansk."
Haidai added that dozens of pieces of Russian heavy military equipment had been brought into the region and had already been deployed on the battlefield.
Russian forces captured most of Severodonetsk weeks ago, but Ukrainian forces have prevented them from completely securing the city. Russian-backed separatists have fought for control of Severodonetsk since 2014.
After Ukrainian troops forced Russian forces from its second-largest city of Kharkiv last month, Moscow intensified attacks on the city Monday with shelling on its outskirts. Officials said the Kremlin did not attempt a ground attack to break through Ukraine's defensive lines.
Britain's Defense Ministry said Monday that Russia has failed to gain full air superiority and has "operated in a risk-averse style, rarely penetrating deep behind Ukrainian lines." It said the failure has added to the slow progress of its ground forces.
"In the conflict to date, Russia's air force has underperformed," the ministry said in a tweet. "Its failure to consistently deliver air power is likely one of the most important factors behind Russia's very limited campaign success."
Meanwhile, European Union foreign policy head Josep Borrell said Monday the Kremlin's blockade of Ukrainian grain exports is a "real war crime."
Borrell pushed back on claims by Russian President Vladimir Putin that Western sanctions against Moscow were the cause of the global food crisis.
"One cannot imagine that millions of tons of wheat remain blocked in Ukraine while in the rest of the world people are suffering hunger," Borrell told EU foreign ministers in a meeting, according to Radio Free Europe/Radio Liberty. "This is a real war crime."
Since February, when Russia's invasion started, Ukraine's grain shipments stalled, leaving more than 20 million tons of grain trapped in silos there.
