Ukraine is getting enough weapons to fight Russia, western defence officials said after a government adviser in Kyiv asked for deliveries of more advanced arms.
Reports suggested that Russia's military may have suffered up to 10,000 dead and the morale of troops was being “closely monitored”.
The numbers, revealed in a pro-Kremlin newspaper, said more were seriously injured.
Intelligence reports indicated that young Russian soldiers struggled to fight against fellow Slavs and civilians, western officials said.
“The morale of Russian forces and their will to keep fighting in a situation where they are fighting in the civilian areas in a fellow Slav country is something that we're watching really carefully,” one said.
Andriy Yermak, chief of staff to President Volodymyr Zelenskyy, speaking from Kyiv during in online seminar, asked the West for weapons of greater sophistication to match Russia’s firepower.
“We cannot win this war without offensive weapons, without medium range missiles that can be a means of deterrence,” he told Chatham House, a London security think tank.
“It's impossible to defend yourself effectively for a long time without a reliable air defence system capable of downing enemy cruise and ballistic missiles at long range, and aircraft.”
Failure to provide weapons would mean that “Ukraine will bleed to death, it will be depopulated,” he said, adding that it would never surrender.
With the war in its fourth week, the Ukrainians had “expended a lot of ordnance” and more than anticipated. There were predictions Ukraine would be fighting an insurgency against the Russians, a western official told the media.
He said there was “no evidence that they are running out,” adding “nor that we would let them”.
“We are seeking to increase the level of our support in all domains to ensure that the Ukrainians have what they need to defend themselves against Russian attack,” he said.
On Monday, the Russian tabloid Komsomolskaya Pravda published a line in a story on casualties that read: “According to Russian Defence Ministry data … 9,861 Russian soldiers had been killed in action and another 16,153 had been wounded.”
The western official agreed that the figures appeared accurate. “This is a level of casualties that has not been experienced really since the Second World War,” he said.
Mr Yermak suggested that because Moscow had failed in its strategic objectives, it was seeking to destroy Ukraine.
“The aggressor is trying to provoke a humanitarian disaster, destroy critical civilian infrastructure and industrial capacity of our country,” he said, adding they had set “no limit to the number of victims”.
In a plea for fighter jets, he said that “Ukraine is the shield of Europe but it stands alone in protecting Europe”.
“To be frank with you, the situation is very difficult. It is dire because Russia will have 10 tanks where we have only one, but in many wars, it is the invincible spirit of the military that prevails over the outnumbering enemy,” he said.
Mr Yermak said that Ukraine wanted to end the fighting so that “as many civilians as possible survive”.
He also suggested that Kyiv would be willing to discuss the territories seized by Russia in 2014.
“We are prepared to negotiate the peace agreement and we are prepared to talk about Crimea and about Donbas,” he said.