Russia has likely moved multiple air defense assets, including SA-15 and SA-22 missile systems, to Snake Island in the western Black Sea, the U.K. Ministry of Defense said Monday in an intelligence update.

The move follows the loss of the Russian warship Moskva, the flagship of Russia's Black Sea Fleet.

"It is likely these weapons are intended to provide air defence for Russian naval vessels operating around Snake Island," the ministry added. "Russia's activity on Snake Island contributes to its blockade of the Ukrainian coast and hinders the resumption of maritime trade, including exports of Ukrainian grain."

Russian forces captured Ukraine's Snake Island in the early days of the invasion, memorably when Ukrainian soldiers defending the tiny islet told an attacking Russian warship to "go f--- yourself." Ukrainian troops have failed in their attempts to retake the previously inconsequential territory.

Meanwhile, in eastern Ukraine's contested Donbas region, heavy fighting continues in the war-torn city of Sieverodonetsk, according to the ministry.

"Russian forces continue to push towards Sloviansk as part of their attempted encirclement of Ukrainian force," the ministry said.

And in the Ukrainian capital, Kyiv, Russian air-launched cruise missiles struck rail infrastructure Sunday in the early morning hours, "likely in an attempt [to] disrupt the supply of Western military equipment to frontline Ukrainian units," according to the ministry.

After five weeks of relative calm in Kyiv, Russian rockets hit Ukraine's capital city on Sunday as Russian President Vladimir Putin warned of strikes on "new targets" if the United States goes through with plans to supply Ukraine with longer-range missiles.

Ukrainian Deputy Minister of Defense Hanna Maliar said the war is still in its "hot phase" and "capturing Kyiv is still Russia’s main goal."

An ABC News crew visited Kyiv's Darnytskyy district, where several Russian cruise missiles slammed into a railway repair plant. One building was still on fire when the ABC News crew arrived. Nearby, another missile strike left a creater on a cement path.

It took hours before Ukrainian authorities permitted media access to the site, saying the area needed to be cleared for safety first.

The Russians claimed the attack in Darnystskyy destroyed military vehicles and armaments. Ukrainian officials said the missiles hit a railway repair plant where no tanks were stored.

Speaking on Russian TV on Sunday, Putin issued a warning to the West on supplying the Ukrainians with high-powered rocket systems. He said if the West carried through with it, Russia would hit "new targets they had not attacked before."

President Vladimir Putin warned that Russian forces would strike new targets if the West began supplying Ukraine with longer-range missiles.

"But if they [missiles] are actually delivered, we will draw appropriate conclusions and apply our own weapons, which we have in sufficient quantities to carry out strikes on targets we aren't striking yet," Putin told Rossyia 1 TV Channel in an interview on Sunday.

-ABC News’ Tanya Stukalova and Tomek Rolski

Two Reuters journalists were injured and their driver killed in an attack in eastern Ukraine Friday, Reuters said.

Photographer Alexander Ermochenko and cameraman Pavel Klimov were traveling into Sievierodonetsk when they came under fire on a Russian-held part of the road, according to Reuters. Klimov was treated for an arm fracture and Ermochenko for a small shrapnel wound at a nearby hospital.

Their driver, who has not been identified, had been assigned to Reuters by Russia-backed forces.

"Reuters extends its deepest sympathies to the family of the driver for their loss," a Reuters spokesperson said in a statement.

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