Troops from several elite Russian military units based along NATO’s borders have suffered casualty rates of 30 to 40 percent in Moscow’s ongoing invasion of Ukraine, one European defense official has said, with experienced troops now being replaced by recently mobilized reserves.

NATO defense and intelligence officials are keeping a close eye on the performance and fortunes of key Russian units—expected to be in the vanguard of any future Russian offensive against NATO in the Baltic region—that traditionally face alliance troops across the 755-mile NATO-Russia border; soon to be extended once Finland joins the transatlantic bloc.

A European defense official who spoke with Newsweek on the condition of anonymity given the sensitivity of NATO intelligence-gathering methods revealed that groups deployed to Ukraine from three Russian formations based close to the border with Estonia and Latvia have lost between one-third and one-half of their original personnel since the invasion began on February 24.

The 76th Air Assault Division—part of the famed Russian Airborne Forces, or VDV—is based in the city of Pskov, only 40 miles from the Estonian border. Members of the 76th were responsible for “cleansing operations” in the Kyiv suburb of Bucha, in which many civilians were tortured and executed. Moscow still denies that the well-documented atrocities across occupied regions took place.

“Sub-units of the 76th Air Assault Division have been located in the areas of heavy battles: Kyiv direction, later Izyum and Kherson,” the European defense official told Newsweek. “It is highly likely the division had heavy losses.”

“We can assess 30 to 40 percent of the troops that deployed to the Ukraine war are wounded, missing or killed. We assess that in their home base in Pskov and Cherekha mainly conscripts and some contractors/officers are left,” they said.

There have already been rumblings about the poor fortunes of the 76th. In May, Major General Veiko-Vello Palm, the deputy commander of the Estonian Defence Forces, told Newsweek that the formation “has been mainly conducting funeral processes for the soldiers that have been killed in Ukraine.”

Newly-mobilized personnel forming two battalions—generally between 700 and 900 troops—began training at the 76th’s home base at the start of October, the European official added. By the end of November, these units were assessed to be heading to Ukraine or towards Ukrainian borders.

Two other units are thought to have suffered similar casualty rates. The Pskov-based 2nd Special Purpose Brigade—a Spetsnaz special forces formation and part of the GRU military intelligence agency—was awarded honorary “Guards” status by Russian President Vladimir Putin in July for its actions in Ukraine.

The 25th Separate Guards Motor Rifle Brigade, a Russian Army formation based in the city of Luga, was reportedly badly mauled in the Ukrainian counteroffensive in northeastern Kharkiv Oblast, which stunned Moscow and liberated more than 500 settlements and 4,600 square miles.

Both the 2nd and 25th brigades “had heavy losses,” the European defense official said. “We assess that possible losses are 30 to 40 percent.”

The demands of Russia’s war in Ukraine have forced Moscow to redeploy troops from border bases to the battlefields. In September, Foreign Policy cited three unnamed senior European defense officials who said that around 80 percent of the 30,000 Russian troops previously positioned along Baltic and Finnish borders had been diverted to Ukraine.

In October, Kai Sauer—the under-secretary of state for foreign and security policy at the Finnish Foreign Ministry—told Newsweek it could take “one to three years, really depending on how the war progresses, and also how the training of the new recruits takes place,” for Russian border units to return to pre-invasion strength.

Sauer said Helsinki is closely following the performance of units traditionally positioned along Finland’s borders, some of which he said have taken “severe casualties.”

To date, Ukraine claims to have inflicted more than 90,000 Russian military casualties since February 24. Kyiv closely protects its own casualty figures as a military secret.

European Commission President Ursula von der Leyen drew ire from Ukraine this week when she said that “20,000 civilians and more than 100,000 Ukrainian military officers” have been killed in the invasion so far. A spokesperson later said the 100,000 figure also included those injured, and that the figure came from unspecified “external sources.”

Chairman of the Joint Chiefs of Staff Mark Milley said in November that around 100,000 troops have died on both the Russian and Ukrainian sides.

Newsweek reached out to the Russian Defense Ministry for comment.