Russian President Vladimir Putin on Monday recognised two rebel Ukrainian regions that have been fighting a deadly insurgency since 2014.

He then signed two decrees ordering Russian "peacekeepers" to support the regions' claims.

Ukrainian Defence Minister Oleksiy Reznikov posted an emotional message on his ministry's website telling his troops to be ready for war.

"There will be hardship. There will be losses. We will have to endure pain, overcome fear and despair," Reznikov said.

But he also promised "certain victory" in the face of the Russian threat.

"The Kremlin has taken another step to reviving the Soviet Union," Reznikov said.

"Yesterday, he showed his real face," he added, in reference to Putin.

"The face of a criminal who wants to hold the whole free world hostage."