Russian forces captured territory along a frontline river in eastern Ukraine and intensified pressure on two key cities on Monday, after Ukrainian President Volodymyr Zelenskiy predicted Moscow would escalate attacks ahead of an EU summit expected to welcome Kyiv's bid to join the bloc.

The governor of Ukraine's Luhansk region, scene of the heaviest Russian onslaughts in recent weeks, said the situation was "extremely difficult" along the entire frontline there as of Monday evening.

"The Russian army has accumulated sufficient amount of reserves to begin a large-scale offensive," the governor, Serhiy Gaidai, said on Ukrainian television.

He said Russian forces controlled most of the city of Sievierodonetsk, apart from the Azot chemical plant, where hundreds of civilians have been sheltering for weeks, and the road connecting Sievierodonetsk and its sister city Lysychansk to the city of Bakhmut was under constant shell fire.

"Lysychansk has been suffering from massive Russian shelling all day. It is impossible to establish the number of casualties as of yet," Gaidai said, adding that the shelling has been perhaps the heaviest the city had yet experienced.

Even so, the Russians had yet to complete an encirclement of Ukrainian forces, who were inflicting "significant losses" on them," he said.

Moscow's separatist proxies claimed to have captured Toshkivka, a town on the mostly Ukrainian-held western bank of the Siverskyi Donets river, south of Sievierodonetsk, which has become the main battlefield city in recent weeks.

Gaidai earlier acknowledged that a Russian attack on Toshkivka had "had a degree of success" and said Russian forces were also trying to gain a foothold near Ustinovka, a village further north along the river. They were bringing a huge amount of heavy equipment there including tanks, he said.

He confirmed Russia's claim to have captured Metyolkine.

On Sunday, in his customary night-time video address to the nation, Zelenskiy said an intensification of Russian attacks was to be expected this week.

"We are preparing. We are ready," he said.

At a summit on Thursday and Friday, European Union leaders are expected to give their blessing to Ukraine becoming an official candidate to join, a decision that will be marked as a triumph in Kyiv.

"I think that's very likely it would happen," U.S. President Joe Biden told reporters when asked on Monday if he felt Ukraine would become an EU member.

Ukraine applied for EU membership just four days after Russia's Feb. 24 invasion. Though membership would take years to attain, for the EU to reach deep into the heart of the former Soviet Union would bring about one of Europe's biggest economic and social transformations since the Cold War.

ATTRITIONAL PHASE

The war has entered a brutal attritional phase in recent weeks, with Russian forces concentrating their overwhelming artillery firepower on a Ukrainian-controlled pocket of the Donbas, which Moscow claims on behalf of separatists.

Ukrainian officials reported three civilians deaths in Russian shelling in the Donetsk region on Monday and another three in shelling in the Kharkiv region.

In Odesa, Ukraine's biggest Black Sea port, a Russian missile attack destroyed a food warehouse on Monday, the Ukrainian military said. No civilians were reported killed.

Odesa has come under sporadic bombardment during the war and is blockaded by the Russian navy.

The Russian-installed leader of Crimea, which Russia annexed from Ukraine in 2014, said Kyiv had struck Black Sea drilling platforms owned by a Crimean oil company. Three people were wounded, and a search was under way for seven workers, he said on Telegram.

Russia's RIA Novosti news agency said the platforms were located 71 km (44 miles) from Odesa. Reuters was unable to immediately verify the reports.

Washington and its European allies have provided weapons and financial assistance to Ukraine but have avoided direct involvement in the conflict. Some American citizens, however, have volunteered to fight alongside Ukrainian forces.

On Monday, the Kremlin said Americans captured in Ukraine were "mercenaries" who had shot at Russian servicemen and were not covered by the Geneva convention, RIA reported.

It quoted Kremlin spokesman Dmitry Peskov as saying they should take responsibility for their "crimes".

Russian media outlets on Friday broadcast images and brief interviews with two captured Americans they identified as Andy Huynh, 27, of Hartselle, Alabama and Alexander Drueke, 39, of Tuscaloosa, Alabama.

This month, two British citizens and a Moroccan were sentenced to death by a separatist court after being caught fighting for Ukraine.

RIA also quoted the Kremlin as saying that American basketball player Brittney Griner, who has been detained in Russia since February, faces criminal prosecution.

Russia has previously said it had detained Griner, a seven-time WNBA All-Star player, for possession of vape cartridges containing hash oil.

International concern has focused on trying to restore Ukrainian exports of food, now shut by a de facto Russian blockade. Ukraine is one of the world's leading sources of grain and food oils, leading to fears of global shortages and hunger.

EU foreign policy chief Josep Borrell called the grain blockade "a real war crime". It was "inconceivable .... that millions of tonnes of wheat remain blocked in Ukraine while in the rest of the world people are suffering hunger".

Russia blames the food crisis on Western sanctions curbing its own exports.

The war has also disrupted energy markets, including Russian shipments of oil and gas to Europe, still the continent's main source of energy and Moscow's primary income source. Moscow blames EU sanctions for a decline in gas volumes, saying they prevented it from restoring pipeline pumping equipment.

Moscow meanwhile threatened to retaliate against EU member Lithuania for banning transport of basic goods to Kaliningrad, a Russian outpost on the Baltic Sea surrounded by EU territory. The ban, which took effect on Saturday, blocks shipments of coal, metals, construction materials and advanced technology.

Russia's foreign ministry summoned Lithuania's top diplomat and demanded Vilnius reverse the "openly hostile" move immediately, or else Russia "reserves the right to take actions to protect its national interests." Lithuania said it was required to enforce the ban under EU sanctions.