KYIV, Ukraine (AP) — The Russian military said Wednesday that it used long-range missiles to destroy a depot in the western Lviv region of Ukraine where ammunition for NATO-supplied weapons was being stored.

Those strikes came as fighting raged for the city of Sievierodonetsk in the eastern Donbas area, the key focus of Russia’s offensive in recent weeks.

Russia-backed separatists accused Ukrainian forces of sabotaging an evacuation of civilians from the city’s besieged Azot chemical plant, where about 500 civilians and an unknown number of Ukrainian fighters are believed to be sheltering. It wasn’t possible to verify that claim.

A humanitarian corridor from the Azot plant had been announced a day earlier by Russia.

The Ukrainian governor of Luhansk, Serhiy Haidai, on Wednesday refused to comment on Russian statements regarding a humanitarian corridor, but told The Associated Press that “heavy fighting in Sievierodonetsk continues today as well.” The situation in the city is getting worse, Haidai admitted, because the Russian forces have more manpower and weapons.

“But our military are holding back the enemy from three sides at once,” Haidai said. “The enemy is advancing because of significant advantage in artillery and people, but the Ukrainian army is holding on to its positions in the city.”

Russian Defense Ministry spokesman Igor Konashenkov said that Russian forces used high-precision Kalibr missiles to destroy the depot near the town of Zolochiv, which is in the Lviv oblast near the border with NATO member Poland.

Konashenkov said M777 howitzers, a type supplied by the United States, were being stored there. He said that Russian airstrikes also destroyed Ukrainian “aviation equipment” at a military aerodrome in the southern Mykolaiv region, he said.

The strikes came as Ukraine keeps up its pressure on Western countries to deliver more arms, and as NATO countries pledge more heavy weapons for Ukraine.

In recent days, Ukrainian officials have spoken of the heavy human cost of the war, with the fierce fighting in the east becoming an artillery battle that has seen Kyiv’s forces outgunned and outnumbered.

“The losses, unfortunately, are painful, but we have to hold out,” Ukrainian President Volodymyr Zelenskyy said late Tuesday in his nightly video address. “The more losses the enemy suffers there, the less strength it will have to continue the aggression. Therefore, the Donbas is key to determining who will dominate in the coming weeks.”

Mykhailo Podolyak, an adviser to Zelenskyy, tweeted Wednesday that he gets a daily message from the Ukrainian defenders there saying: “We are holding on, just say: when to expect the weapons?” He said that is the same message he has for NATO leaders.

Meanwhile, the deputy chair of Russia’s Security Council and former president Dmitry Medvedev suggested that Russia appears intent on the destruction of its neighbor.

In a Telegram post, he wrote that he saw reports that Ukraine wants to receive liquefied natural gas in a deal from its “overseas masters” with payment due in two years.

He added: “But there’s a question. Who said that in two years Ukraine will even exist on the map of the world?”

Medvedev, the former president, has been making harsh statements against Ukraine and the West since the war began on Telegram.

In other developments, a U.N. delegation investigating war crimes in Ukraine has visited areas of the country which were held by Russian troops and says there is evidence which could support war crimes allegations.

The delegation chaired by Erik Møse, a Norwegian judge, visited sites including the Kyiv suburbs of Bucha and Irpin, where Ukrainian authorities have accused Russia of mass killings of civilians.

“At this stage we are not in a position to make any factual findings or pronounce ourselves on issues of the legal determination of events,” Møse said.

“However, subject to further confirmation, the information received and the visited sites of destruction may support claims that serious violations of international human rights law and international humanitarian law, perhaps reaching war crimes and crimes against humanity, have been committed in the areas,” he said.

With Ukrainian and international organizations investigating war crimes cases, Møse expressed concern at the risk of investigations “overlapping” or causing witnesses more trauma by going over the same events repeatedly.