Russia will take years to replace nuclear-capable bomber planes that were hit in Ukrainian drone strikes last weekend, according to Western military aviation experts, straining a modernisation programme that is already delayed.

Satellite photos of airfields in Siberia and Russia's far north show extensive damage from the attacks, with several aircraft completely burnt out, although there are conflicting versions of the total number destroyed or damaged.

The United States assesses that up to 20 warplanes were hit - around half the number estimated by Ukrainian President Volodymyr Zelenskiy - and around 10 were destroyed, two U.S. officials told Reuters.