CNN  — 

Thousands of Russian troops have been killed in Ukraine since Russia’s invasion began nearly one month ago, US and NATO officials told CNN this week, and Russia is now struggling to resupply those forces as it faces sagging troop morale and fierce Ukrainian resistance.

Russia’s offensive to capture Kyiv has largely stalled, NATO officials said, and on Thursday Ukraine said it launched a counteroffensive aimed at gaining decisive control of the city’s suburbs.

US and allied intelligence assessments vary widely as to exactly how many Russian forces have been killed to date, sources familiar with the intelligence tell CNN. But even the lowest estimates are in the thousands.

One such assessment found that approximately 7,000 Russian troops have been killed so far, said one of the sources. But that figure, first reported by The New York Times, is on the higher end of US estimates, which vary because the US and its allies have no precise way of counting casualties. Some estimates place the number of Russian troops killed in Ukraine at about 3,000, whereas others suggest more than 10,000 have been killed.

So far, the number has been calculated largely via open source reporting from non-governmental organizations, the Ukrainian government, commercial satellite imagery, and intercepted Russian communications. US officials have also extrapolated numbers of dead based on the number of Russian tanks that have been destroyed, the sources said.

Regardless of the precise number, US and western intelligence officials have observed that Russia is having difficulty replacing its forces, which is having a significant impact on Russian troop morale, senior NATO officials said on Wednesday.

Smoke rises from a Russian tank destroyed by the Ukrainian forces on the side of a road in Lugansk region on February 26, 2022.

“It becomes more evident every day that Putin gravely miscalculated,” a senior NATO intelligence official told reporters at NATO headquarters on Wednesday night, speaking on the condition of anonymity to disclose sensitive assessments. “Russia continues to face difficulties replacing its combat losses, and increasingly seeks to leverage irregular forces, including Russian private military corporations and Syrian fighters.”

A senior NATO military official echoed that assessment, saying that “we can assess that more private military companies will be engaging” in the conflict soon. But in general, he said, the losses have had “a bad effect on the morale of the troops.”

“We can see [Putin] miscalculated the resilience and the resistance of the Ukrainians,” the NATO military official said. “That is a fact. He did not see that. And that is a big surprise for him. And therefore he has had to slow down.”

Flagging Russian morale

The NATO intelligence official added, citing the Ukrainian General Staff, that “Russian servicemen are increasingly refusing to travel to Ukraine, despite promises of veteran status and even higher salaries.” He noted that NATO expects that “the reportedly high Russian casualties will also stir some reaction in Russia, as the Russian people eventually become aware of the extent of their losses.”

A senior US defense official told reporters Thursday that the Pentagon has anecdotal evidence that Russian morale is flagging.

“We don’t have insight into every unit and every indication. But we certainly have picked up anecdotal indications that morale is not high in some units,” the official said. “Some of that is, we believe, a function of poor leadership, lack of information that the troops are getting about their missions and objectives, and I think disillusionment from being resisted as fiercely as they have been.”

In some instances, Russian troops have simply abandoned broken down vehicles in the field, walking away and leaving behind tanks and armored personnel carriers, according to two US officials.

A congressional source briefed on the intelligence similarly said the US has assessed that there appears to be a gap between what Russian troops were prepared for and what they actually encountered. Many Russians captured so far have said they did not expect, for example, that they would be fighting a war in Ukraine, and believed they were just part of a military exercise.

The Russian military leaders’ commitment, however, appears to still be high, the congressional source said.

A destroyed tank sits on a street after battles between Ukrainian and Russian forces on a main road near Brovary, north of Kyiv, Ukraine, Thursday, March 10, 2022.

Last week, Lt. Gen. Scott Berrier, director of the Defense Intelligence Agency, told a congressional committee in a public hearing that the US intelligence community’s assessment of Russian troop deaths was between 2,000 and 4,000. He said the assessment was made with low confidence and based on both intelligence sources and open-source material.

US and western intelligence officials broadly acknowledge that the will to fight is often difficult to measure and it is unclear how much sagging morale has contributed to Russia’s sluggish progress on the battlefield. But open-source reporting for weeks has documented signs of discontent and low morale amongst ground troops, and one official speculated that one of the reasons Russian generals have been operating in higher-risk, forward operating positions is an effort to gin up flagging troops.

Western officials say at least three Russian generals have been killed by Ukrainian forces since the war began.

The problem may also extend to Russia’s elite air units, the official said.

“They’ve lost a bunch of planes,” this person said. “That really affects pilot morale.”

Russia is also behind in its desired timeline, the senior NATO military official said on Wednesday. Putin was hoping to expand Russian control over Ukraine all the way west to the Moldovan border by now, the official said, in order to link up with more Russian troops and attempt to encircle Kyiv.

There are pro-Russian troops stationed in Transnistria – a breakaway state in Moldova – who “are in a way, prepared” to join the war, the official said. But they have not yet done so because the regular Russian forces have not yet made substantial progress westward, he said.

Despite all of the losses, the senior NATO intelligence official said the alliance considers that Putin is still “unlikely to be deterred, and may instead escalate. He likely remains confident that Russia can militarily defeat Ukraine.”

CNN’s Barbara Starr contributed to this report.