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 Defense Ministry says the campaign has been successful.

A senior U.S. defense official said the U.S. had indications that in the last 24 hours Moscow had become frustrated by slow progress, caused by an unexpectedly strong Ukrainian defense and failure to achieve complete air dominance. Still, with Russian forces closing on the capital Kyiv amid fierce street battles on Saturday, the official added Russia so far had committed only about 50% of its available firepower to the war.