The information lays out a scenario where troops would cross into Ukraine from Crimea, the Russian border and via Belarus, with about 100 battalion tactical groups — potentially around 100,000 soldiers — deployed for what the people described as an operation in rough terrain and freezing conditions, covering extensive territory and prepared for a potentially prolonged occupation.

Two of the people said about half that number of tactical groups was already in position and that any invasion would be backed up by air support.

The two people said that Moscow had also called up tens of thousands of reservists on a scale unprecedented in post-Soviet times. They explained the role of reservists in any conflict would be to secure territory in a later phase after the tactical battalions paved the way. Russia hasn’t publicly announced any major call-up of reservists.