NATO estimates that up to 15,000 Russian troops have been killed since the Kremlin's attack on Ukraine began last month, with as many as 40,000 dead, wounded, taken prisoner or missing.

The alliance arrived at those figures based on information from Ukrainian officials, Western intelligence and information gleaned from Russia through official channels or unintentionally, a senior military official from NATO told The Wall Street Journal and The Associated Press.

NATO estimates that roughly 7,000 to 15,000 Russian soldiers have been killed since the attack started on Feb. 24, a major blow to Moscow, which sought to decapitate the Ukrainian government in a matter of days.

But the Russians were quickly mired by fierce Ukrainian opposition and issues with supplies, with the campaign now hitting four full weeks of fighting and NATO warning it is "rapidly approaching" a stalemate.

Ukraine also claims to have killed six Russian generals, while Moscow has only acknowledged one dead.

The new NATO figures mark the first time the alliance has publicly released Russian casualty estimates since the start of the war. U.S. officials have not provided public estimates of Russian or Ukrainian casualties, pointing to the questionable reliability of such information and a fast-moving and ever-changing conflict.

Russia has also closely guarded information on its casualties. The last time it acknowledged such information was on March 2, when it said almost 500 soldiers were killed and nearly 1,600 wounded.

On the Ukrainian side, President Volodymyr Zelensky said Wednesday that thousands of his people had been killed, including at least 121 children.

Zelensky said on March 12 that about 1,300 Ukrainian troops had been killed in action.

The Russian troop death toll has already exceeded the U.S. militaries losses in the first 18 years of the Afghanistan war, when just over 7,000 troops died.

The Kremlin, despite sending more than 150,000 troops into Ukraine, has made limited progress with its ground forces in recent weeks.

The Russian troops have been slowed or stopped by Ukrainian units using hit-and-run tactics and Western-supplied weapons, forcing them to destroy cities from afar using bombs and heavy shelling.

Moscow has lost more than 10 percent of its combat force in Ukraine, with major issues with food, fuel and cold weather gear shortages, a senior U.S. defense official told reporters Tuesday.

Ukrainians forces, meanwhile, have continued to defend several major cities including the capitol of Kyiv and have started to move to take back territory Russians had gained in recent days.