Russia’s plans for Ukraine face rapidly rising costs due to delays caused by tougher-than-expected resistance from forces on the ground, even as its military retains overwhelming advantages.

A person familiar with Russian planning said the military would have hoped for faster progress. The Kremlin has declined to comment on details of the military operation and its defence ministry says the campaign has been successful.

A senior US defence official said the US had indications that in the past 24 hours Moscow had become frustrated by slow progress caused by an unexpectedly strong Ukrainian defence and failure to achieve complete air dominance. Still, the official added Russia so far had committed only about 50% of its available firepower to the war...