Kursk incursion: How significant is Ukraine's advance into Russian territory?

"As ever, a developing situation, so we should be wary of grand predictions," Matthew Savill, military sciences director at the Royal United Services Institute (RUSI) thinktank, told Sky News. The penetration seems to be 10km to 20km and may involve several thousand troops, he said, but added that the situation is very confusing and caution is required. "It's unlikely that the plan is to seriously take Kursk itself, or to try and hold vast swathes of Russian territory," he said. "The Ukrainians don't have much in the way of reserves, whereas the Russians do and sustaining a medium-sized force into Russia right now would be tough for the Ukrainians."