Russia's invasion of Ukraine has caused more than two million military casualties, with Moscow's forces bearing the brunt of the losses, according to a study published Wednesday by a US think tank.

"Combined Russian and Ukrainian casualties have exceeded 2 million," the Center for Strategic and International Studies (CSIS) said.

It estimated that between 400,000 and 450,000 Russians have been killed since Moscow invaded Ukraine in February 2022, out of a total of 1.4 million casualties, who were killed, wounded and missing, among Russian troops.

Ukrainian forces have meanwhile suffered between 525,000 and 625,000 casualties and between 125,000 and 150,000 fatalities during the same period, CSIS said.

"Russian fatalities in Ukraine are more than four times greater than all US fatalities in all wars combined since World War II," while the ratio of Russian to Ukrainian casualties has likely risen to around 8 to 1 in the first half of this year, it added.