Russia launched an intense missile and drone barrage at the Ukrainian capital Kyiv in the early hours of Friday, killing at least six people, Ukrainian officials said, as powerful explosions reverberated across the country.

Russian President Vladimir Putin "will have to respond" to Ukraine's recent drone attack on military airfields, U.S. President Donald Trump said on June 4 after holding a call with the Russian president.

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.