The Russian military says it has used long-range missiles to destroy a depot in the western Lviv region of Ukraine where ammunition for NATO-supplied weapons was being stored.
Russian Defence Ministry spokesman Igor Konashenkov said that Russian forces used high-precision missiles to destroy the depot near the town of Zolochiv, which is near the border with NATO member Poland.
Mr Konashenkov said M777 howitzers, a type supplied by the United States, were being stored there.
He added that Russian air strikes also destroyed Ukrainian "aviation equipment" at a military aerodrome in the southern Mykolaiv region.
The strikes come as fighting raged for the city of Sievierodonetsk in the eastern Donbas area, the key focus of Russia's offensive in recent weeks.
Ukraine continues to keep up pressure on western countries to deliver more arms, and NATO countries have pledged more heavy weapons for Ukraine.
Watch: Ukraine continues appeals for missile defence systems.
Russia-backed separatists accused Ukrainian forces of sabotaging an evacuation of people 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 was not 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, has refused to comment on Russian statements regarding a humanitarian corridor, but said that "heavy fighting in Sievierodonetsk continues today as well".
The situation in the city is getting worse, Mr Haidai admitted, because the Russian forces have more manpower and weapons.
Watch: Poland and Ukraine set for $700m arms deal.
"But our military are holding back the enemy from three sides at once," he said. "The enemy is advancing because of a significant advantage in artillery and people, but the Ukrainian army is holding on to its positions in the city."
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," said Ukrainian President Volodymyr Zelensky on 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 Ukraine's president, tweeted on 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.