A top ally of Russian President Vladimir Putin said the Ukraine war could last for decades, with long periods of fighting interspersed by truces, Russia's RIA news agency reported on Thursday.

It said former President Dmitry Medvedev, deputy chairman of Putin's powerful security council, had spoken during a visit to Vietnam. Medvedev often makes hard line comments and last month described Ukrainian authorities as an infection.

"This conflict will last a very long time, most likely decades," RIA cited Medvedev as saying.

"As long as there is such a power in place, there will be, say, three years of truce, two years of conflict, and everything will be repeated," he continued, reiterating Moscow's claim that Ukraine is a Nazi state.