The number of Russians killed or injured in Ukraine has reached 188,000, according to US intelligence, it has been reported.
Vladimir Putin's forces are also said to have seen 2,000 of their tanks destroyed or captured since the invasion began nearly a year ago.
The new figures were shared at a Western summit on Thursday held at Ramstein Air Base in southern Germany.
They dwarf the 100,000 "dead, injured, or have deserted" estimate given by UK Defence Secretary Ben Wallace last month.
Top US General Mark Milley said in November said somewhere just over that figure are thought to be casualties but added Ukraine's military had "probably" lost similar numbers.
In relation to equipment, Wallace said Russia had lost 4,500 armoured vehicles, 63 fixed-wing aircraft, 70 helicopters, 150 unmanned aerial vehicles (UAVs), 12 naval vessels, and over 600 artillery systems.
However, the Sun reports Milley told summit attendees this week the personnel losses were "significantly well over" the number he gave previously.
He said: “They have really suffered a lot...I would say significantly well over 100,000."
Representatives from around 50 countries met at the main US airbase in Europe, with White House defence secretary Lloyd Austin looking to drum up further support for Ukraine.
Speaking at the event, Milley said: “They have suffered a tremendous amount of casualties in their military, that includes the military and the Wagner mercenary group."
The Wagner Group are considered Putin's 'private army' and are reported to number around 8,000 soldiers.
It comes after Boris Johnson said Putin will not use nuclear weapons, comparing the Russian president to "the fat boy in Dickens" who wants to "make our flesh creep" with hollow threats.
The former prime minister was speaking at a breakfast event on Ukraine at the World Economic Forum in Davos this week, where he was introduced as a "legendary figure" in Ukraine.
Asked what he thought was going through Putin's mind, Johnson warned against spending too much time obsessing about the Russian leader or worrying about the escalation of the conflict.
Johnson said: "Putin wants to present it as a nuclear stand-off between Nato and Russia.
"Nonsense. He's not going to use nuclear weapons, OK.
"He's like the fat boy in Dickens, he wants to make our flesh creep.
"He wants us to think about it. He's never going to do it."
