Russian government spokesperson Dmitry Peskov admitted late Thursday that Russia has suffered "significant losses" of troops amid its brutal invasion of Ukraine.

"This is a huge tragedy for us," Peskov told Sky News reporter Mark Austin during a live interview.

The official also denied that Russia's invasion of Ukraine had become a humiliation for Kremlin amid Ukrainian forces' fierce resistance.

In a statement on March 25, Moscow announced that more than 1,000 Russian soldiers have died during the military operation in Ukraine.

Nearly 4.25 million Ukrainians have fled the country during Russia's invasion, while a further 7.1 million are thought to be internally displaced within Ukraine, the United Nations said on Tuesday.

Many in Ukraine are bracing for further Russian bombardments, especially in the east and south, and air raid sirens rang out overnight.

The full nature of the killings in Bucha and other areas from which Russian troops have withdrawn is still being pieced together.

Ukrainian officials say over 400 civilian bodies have been recovered from the wider Kyiv region, many of whom have been buried in mass graves.