Russia started its "special military operation" in Ukraine to prevent incursion on its territory, and it was "the only right decision," President Vladimir Putin said on Monday.

Addressing the 77th Victory Day at the Red Square in Moscow, marked to celebrate Soviet Union's victory over Nazi Germany in World War II, Putin said the military is fighting for the people in Donbass, and for the security of Russia.

"Last year we suggested NATO countries to sign a security treaty, but they did not want to hear us, they had completely different plans, and an attack on Crimea was being prepared," he said. "The alliance began military development of the territories adjacent to us ... a clash with neo-Nazis was inevitable."

The Russian president also observed a minute of silence in honor of those who "gave their lives for the Motherland."

"The death of every soldier is a grief for all of us, we will do everything to help the families and children of servicemen killed in Ukraine," Putin said.

Moscow calls its war on Ukraine, which began in February, a “special military operation” to “demilitarize” and “denazify” Ukraine. For Western countries, however, it is a war of aggression and they have imposed severe sanctions in response.

The military action has left thousands dead or wounded, and forced more than 5.7 million people in Ukraine to flee abroad.