Russia will defeat Ukrainian forces in the whole of the eastern Donbas region and is unlikely to withdraw from a vast swathe of land across Ukraine's southern coast, Russia's ambassador to London told Reuters.

Since the Feb. 24 invasion, Russian forces have taken control of a big chunk of territory across Ukraine's southern flank above Crimea, which Russia annexed in 2014, and is slowly pushing Ukrainian forces out of two Russian-backed rebel regions of east Ukraine which it has recognised as independent states.

When asked how the conflict might end, Russian Ambassador Andrei Kelin said Ukraine forces would be pushed back from all of Donbas and that it was difficult to see Russian and Russian-backed forces withdrawing from the south of Ukraine.

"We are going to liberate all of the Donbas," Kelin told Reuters in an interview in his London residence where Winston Churchill used to discuss World War Two strategy with Josef Stalin's ambassador.

"Of course it is difficult to predict the withdrawal of our forces from the southern part of Ukraine because we have already experience that after withdrawal, provocations start and all the people are being shot and all that."

Sooner or later, Kelin said, Ukraine would have to decide: strike a peace deal with Russia or "continue slipping down this hill" to ruin. (Reporting by Jake Cordell, editing by Guy Faulconbridge)