As many as 15,000 Russian troops have been killed since the invasion of Ukraine began four weeks ago, according to NATO’s first estimate.

Russia has suffered 30,000 to 40,000 battlefield casualties, including 7,000 to 15,000 killed, a senior NATO military officer said in a briefing Wednesday from the alliance’s military headquarters in Belgium. 

Also Wednesday, Secretary of State Antony Blinken said the U.S. has determined Russian military forces have committed war crimes in Ukraine.

The senior military officer, speaking on condition of anonymity under ground rules set by NATO, said the estimated deaths are based on information from the Ukrainian government, indications from Russia and open-source information. The officer said the number of fatalities came from a calculation of three wounded soldiers for every soldier killed. Casualties include killed, wounded or missing in action as well as those taken prisoner.

Earlier this week, a senior Pentagon official estimated the Russian military had lost more than 10% of the combat force in Ukraine. 

Russia is beginning to dig into defensive positions outside Kyiv in the face of fierce Ukrainian opposition, a senior Defense official said. The Russian advance from the north remains stalled about 10 miles from the city center, said the official, who spoke on condition of anonymity to discuss intelligence assessments. To the east of Kyiv, the Ukrainians have pushed back the front line about 15 miles to 30 miles outside the city.

The Russians appear to be attacking more aggressively in eastern Ukraine, the official said. Fighting between Ukrainian forces and Russian-backed separatists has been ongoing in the region since 2014.

The Ukraine Defense Ministry claims its forces have driven occupying Russian troops out of Makariv – a small Kyiv suburb that’s crucial because it provides control of a highway to the west. The effort also blocked Russian troops from surrounding Kyiv from the northwest and provided the Ukrainian military with a much-needed success story.