Russia may have suffered between 30,000 and 40,000 battlefield casualties in Ukraine, according to a senior Nato military officer.
The military officer, speaking on condition of anonymity under ground rules set by Nato, added between 7,000 and 15,000 Russians had been killed since it invaded its neighbour on 24 February.
The estimate of those killed is based on information from the Ukrainian government, indications from Russia, and open-source data, Associated Press reported.
It is Nato’s first public estimate of Russian casualties since the beginning of the war.
The US government has largely declined to provide public estimates of Russian or Ukrainian casualties, saying available information is of questionable reliability.
Watch: Mariupol residents suffer continued fighting almost a month since the Russian invasion
The Nato military officer, in a briefing from the alliance’s military headquarters in Belgium on Wednesday, said the estimate of 30,000 to 40,000 Russian casualties is derived from what he called a standard calculation that in war an army suffers three wounded soldiers for every soldier killed.
The officer said the casualties include killed in action and wounded in action, and those taken prisoner or missing in action.
The exact toll on Putin's forces is unknown.
Earlier this week, Western intelligence officials placed the tally of Russian deaths at a "reasonable estimate" of 10,000. If true, this would represent the heaviest number of Russian casualties since the Second World War.
This figure appeared to match a report in a pro-Kremlin Russian newspaper called Komsomolskaya Pravda on Tuesday, which cited Russian Defence ministry data confirming 9,861 troops had died.
However, the death tally was quickly deleted from the article with the newspaper later accusing hackers of planting fake news on its website.
Russia has not officially updated its casualty figures since stating on 2 March that 498 servicemen had been killed and 1,597 wounded.
What is clear, however, is that Russian forces have taken heavy losses in the four weeks since the invasion began. They have been frozen in place for at least a week on multiple fronts and face supply problems and fierce resistance.
As a consequence, Russia has turned to siege tactics and bombardment, causing massive destruction and many civilian deaths.
Despite its losses so far, Russia may still be hoping to make more gains on the battlefield, especially in the east, in territory including Mariupol, which Moscow demands Ukraine cede to Russian-backed separatists.
But in a daily intelligence update, the UK's defence ministry said the entire battlefield across northern Ukraine – which includes huge armoured columns that once bore down on Kyiv – was now "static", with the invaders apparently trying to reorganise.
A veteran aide of president Vladimir Putin has also resigned over the war and left Russia with no intention to return, two sources said on Wednesday, making him the first senior official to break with the Kremlin since Putin launched his invasion a month ago.
The Kremlin confirmed that the aide, Anatoly Chubais, had resigned of his own accord.
In a further sign of growing Ukrainian confidence, on Wednesday a Ukrainian presidential adviser Oleksiy Arestovych said he expected the active phase of the Russian invasion to be over by the end of April as the Russian advance had already stalled in many areas.
Speaking on local television, Arestovych said Russia had already lost 40% of its attacking forces and played down the prospect of Russia waging nuclear war.
Nato will likely decide on Thursday to ramp up military forces on its eastern flank, the head of the alliance said, while also warning Russia against using nuclear weapons.
"I expect leaders will agree to strengthen Nato's posture in all domains, with major increases in the eastern part of the alliance. On land, in the air and at sea," Nato chief Jens Stoltenberg told a news conference ahead of the summit in Brussels on Thursday.