President Vladimir Putin said Monday that Russian forces in Ukraine were defending the Motherland from an "absolutely unacceptable threat," as he opened the annual parade marking victory over Nazi Germany in World War II.

Putin told thousands of troops gathered in Moscow's Red Square that Russian forces in Ukraine were continuing the battle against Nazism, but that it was important "to do everything so that the horror of a global war does not happen again."

Putin made no major announcements in the speech, despite reports in the West that he could use the anniversary to announce an escalation of the conflict in Ukraine or a general mobilisation in Russia.

Instead, Putin looked to rally public support for the campaign by linking the current conflict to what Russians call the Great Patriotic War.

Addressing Russian forces in Ukraine, he said: "You are fighting for the Motherland, for its future, so that no one forgets the lessons of the Second World War."

Putin blamed Ukraine and the West for the conflict, saying Kyiv and its allies had been preparing "an invasion of our historical lands" including in the Russian-speaking Donbas region and in Crimea, annexed by Moscow in 2014.

"An absolutely unacceptable threat to us was being created, directly on our borders," Putin said, pointing to NATO weapons deliveries to Ukraine and the deployment of foreign advisors.

Russia had no choice, Putin said, but to undertake a pre-emptive response to aggression, calling it "the only right decision" for a "sovereign, strong and independent country".

Russia was on Monday marking the 77th anniversary of its defeat of Nazi Germany with parades and marches, including the main celebration in Red Square featuring some 11,000 troops and more than 130 military vehicles.     

Troops parade in Moscow.

Ukraine won't allow Russia to 'appropriate' WWII victory over Nazism: Zelensky  

Meanwhile, Ukraine President Volodymyr Zelensky said Monday that Ukraine would not allow Russia to appropriate victory in World War II.

"Today we celebrate Victory Day over Nazism. We are proud of our ancestors who together with other nations in the anti-Hitler coalition defeated Nazism. And we will not allow anyone to annex this victory. We will not allow it to be appropriated," the Ukrainian leader said. 

Zelensky listed several Ukrainian towns and cities currently under control of invading Russian forces, saying that Ukrainians during World War II had ousted Nazi Germany's forces from these regions.

"The names of these cities inspire us today. They give us faith that we will drive the occupiers from our land," Zelensky said in the video address, listing Mariupol, Kherson, and the Crimean peninsula by name.

"We won then. We will win now," the Ukrainian president added.

Ukraine was invaded by Russia in late February and Moscow claimed its operation was in part to "de-Nazify" the country.

Both Ukraine and Russia have likened actions by the other side's army to those of Nazi Germany, whose defeat by the Soviet Union in 1945 is celebrated in ex-Soviet countries on May 9.