Ukrainian troops defending the eastern city of Sieverodonetsk are "holding," despite attacks in three directions from Russian forces, the U.K. Ministry of Defense said Wednesday in an intelligence update.

"Russia continues to attempt assaults against the Sieverodonetsk pocket from three directions although Ukrainian defences are holding," the ministry said. "It is unlikely that either side has gained significant ground in the last 24 hours."

Sieverodonetsk, an industrial hub, is the largest city still held by Ukrainian troops in the contested Donbas region of Ukraine's east, which comprises the self-proclaimed republics in Luhansk and Donetsk oblasts. In recent days, Russian forces have encircled the city as they advanced in Donbas, creating a pocket that could trap Ukrainian defenders there and in the neighboring city of Lysychansk.

Sievierodonetsk and Lysychansk are the last major cities in the Luhansk area still controlled by Ukraine.

Last week, the U.K. Ministry of Defense said Russian forces had seized most of Sieverodonetsk, but that the main road into the pocket likely remained under Ukrainian control.

With the frontage of the Russian-occupied territories of Ukraine stretching for over 300 miles, "both Russia and Ukraine face similar challenges in maintaining a defensive line while freeing up capable combat units for offensive operations," according to the ministry.

"While Russia is concentrating its offensive on the central Donbas sector, it has remained on the defensive on its flanks," the ministry said in its intelligence update Wednesday. "Ukrainian forces have recently achieved some success by counter-attacking in the south-western Kherson region, including regaining a foothold on the eastern bank of the Ingulets River."

At least three people were killed and six others were injured in the Kharkiv area from ongoing shelling by Russian forces, according to the Kharkiv regional governor, Oleg Synegubov.

-ABC News' Will Gretsky

Oleksiy Danilov, Ukraine's secretary of the National Security and Defense Council, told ABC News that "it will be difficult for Ukraine to win this war without speeding up the supply of modern weapons."

He added, "The country is ready for long-term resistance, because we are fighting for our freedom."

This comes as the Donetsk People's Republic claims an advance in territory.

DPR Foreign Minister Natalia Nikonorova told reporters, "We can say that the allied forces -- the DPR militia and units of the Russian Defense Ministry -- are in control of over 70% of the territory."

There is evidence of Russian vessels departing "from near Ukraine with their cargo holds full of grain," a U.S. Department of State spokesperson told ABC News on Monday night.

The Ukrainian Ministry of Foreign Affairs has reported that Russia seized at least 400,000 to 500,000 tons of grain worth over $100 million, according to the State Department spokesperson.

"Ukraine's MFA also has numerous testimonies from Ukrainian farmers and documentary evidence showing Russia's theft of Ukrainian grain," the spokesperson said.

The news of Ukrainian grain aboard Russian ships partly confirms a recent report by The New York Times that Moscow is seeking to profit off of grain plundered from Ukraine by selling the product while subverting sanctions. Ukraine has already accused Russia of shipping the stolen grain to buyers in Syria and Turkey.

Russia and Ukraine -- often referred to collectively as Europe's breadbasket -- produce a third of the global supply of wheat and barley, but Kyiv has been unable to ship exports due to Moscow's offensive. A Russian blockade in the Black Sea, along with Ukrainian naval mines, have made exporting siloed grain virtually impossible and, as a result, millions of people around the world -- particularly in Africa and the Middle East -- are now on the brink of famine.

-ABC News' Shannon Crawford

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan