Russia asking questions to Ukraine about how they deploy their troops. Kursk is a big gamble as it is, but if it your enemy is potentially bearing in on the capital city then it's not worth a damn.
Ukraine is obviouusly trying to manage this. Last week, Ukraine were insisting they didn't see any build up even after Belarus announced that they'd done it with claims that Ukraine had sent drones over their airspace as justification. Even if Belarusian troops aren't up to scratch, there's a healthy smattering of Wagner PMC in there to make them a trouble if they go in under a legitimate or spurious reason. Ukraine will have to be clearly avoiding anything that brings Belarus into this.
God knows what Ukraine have in terms of defensive lines against invasion from the Belarus-Ukrainian border if any.