The total number of Ukrainian and Russian troops killed or wounded since the war in Ukraine began 18 months ago is nearing 500,000, U.S. officials said, a staggering toll as Russia assaults its next-door neighbor and tries to seize more territory.

The officials cautioned that casualty figures remained difficult to estimate because Moscow is believed to routinely undercount its war dead and injured, and Kyiv does not disclose official figures. But they said the slaughter intensified this year in eastern Ukraine and has continued at a steady clip as a nearly three-month-old counteroffensive drags on.

Russia’s military casualties, the officials said, are approaching 300,000. The number includes as many as 120,000 deaths and 170,000 to 180,000 injured troops. The Russian numbers dwarf the Ukrainian figures, which the officials put at close to 70,000 killed and 100,000 to 120,000 wounded.

But Russians outnumber Ukrainians on the battlefield almost three to one, and Russia has a larger population from which to replenish its ranks.