Nato has estimated that 7,000 to 15,000 Russian soldiers have been killed in four weeks of war in Ukraine.
By way of comparison, Russia lost about 15,000 troops over 10 years in Afghanistan.
A senior Nato military official said the alliance’s estimate was based on information from Ukrainian authorities, what Russia has released — intentionally or not — and intelligence gathered from open sources.
But with Wednesday marking four full weeks of fighting, Moscow is bogged down in a grinding military campaign.
With its ground forces slowed or stopped by hit-and-run Ukrainian units armed with Western-supplied weapons, Russian President Vladimir Putin’s troops are bombarding targets from afar, falling back on the tactics they used in reducing cities to rubble in Syria and Chechnya.
As President Joe Biden left for Europe to meet key allies about new sanctions against Moscow and more military aid to Ukraine, he warned there is a “real threat” Russia could use chemical weapons.
He cited evidence of indiscriminate or deliberate attacks against civilians and the destruction of apartment buildings, schools, hospitals, shopping centres and other sites.
Addressing Japan’s parliament, Ukrainian President Volodymyr Zelensky said thousands of his people have been killed, including at least 121 children.
Still, major Russian objectives remain unfulfilled. The capital, Kyiv, has been bombarded repeatedly but is not even encircled.
Near-constant shelling and gunfire shook the city on Wednesday, with plumes of black smoke rising from the western outskirts, where the two sides battled for control of multiple suburbs.
Mayor Vitali Klitschko said at least 264 civilians have been killed in the capital since the war broke out.
But Ukrainian forces have prevented its fall, thwarting an apparent bid by Moscow to fully secure a land bridge from Russia to Crimea, seized from Ukraine in 2014.
Mr Zelensky said 100,000 civilians remain in a city that had 430,000 people before the war. Efforts to get desperately needed food and other supplies to those trapped have often failed.
He accused Russian forces of seizing a humanitarian convoy. Deputy Prime Minister Iryna Vereshchuk said the Russians were holding captive 11 bus drivers and four rescue workers along with their vehicles.
In the besieged northern city of Chernihiv, Russian forces bombed and destroyed a bridge that was used for aid deliveries and civilian evacuations, regional governor Viacheslav Chaus said.
Kateryna Mytkevich, who arrived in Poland after fleeing Chernihiv, wiped away tears as she spoke about what she had seen. The city is without gas, electricity or running water, said Ms Mytkevich, 39, and entire neighbourhoods have been destroyed.
“I don’t understand why we have such a curse,” she said.
The most recent figure for Ukraine’s military losses came from Mr Zelensky on March 12, when he said that about 1,300 Ukrainian servicemen had been killed in action.
The Nato official said 30,000 to 40,000 Russian soldiers are estimated to have been killed or wounded.
Russia has released very little information on its casualties, saying on March 2 that nearly 500 soldiers had been killed and almost 1,600 wounded.
Ukraine also claims to have killed six Russian generals. Russia acknowledges just one dead general.
The figures from Nato represent the alliance’s first public estimate of Russian casualties since the war began.
The US government has largely declined to provide estimates of Russian or Ukrainian casualties, saying available information is of questionable reliability.