Before Russia's invasion of Ukraine, US intelligence had predicted a blistering assault by Moscow that would quickly mobilise the vast Russian air power that its military assembled in order to dominate Ukraine's skies.

But the first six days have confounded those expectations and instead seen Moscow act far more delicately with its air power, so much so that US officials can't exactly explain what's driving Russia's apparent risk-averse behaviour.

"They're not necessarily willing to take high risks with their own aircraft and...