NATO has released satellite images showing coordinated incursions by Russian self-propelled artillery onto Ukrainian soil, from which they are attacking Ukrainian military targets, providing further confirmation of what has already become quite clear this week: Russia is overtly invading Ukraine.
Here is an image from August 21st, of a convoy of self-propelled artillery moving inside rebel-held territory near Krasnodon in eastern Ukraine:
Self-propelled artillery are not something that Ukraine's separatist rebels could acquire or likely even use on their own. These are large, complex military vehicles that require the coordinated use of support vehicles and lots of training; training that US officials say they believe the rebels do not have, suggesting that the artillery is manned by uniformed Russian soldiers.
Here is Russian self-propelled artillery lining up in firing position in the same region two days later, on August 23rd:
Russia is moving its artillery, as well as troops and possibly tanks, into eastern Ukraine to fight alongside the pro-Russia separatist rebels against Ukrainian military forces. It is not clear precisely what Russian President Vladimir Putin is hoping to achieve — does he want to annex eastern Ukraine as he did in Crimea, or just keep the rebels from being defeated? — but the decision to so overtly invade is a major, major escalation of his months of supporting separatist rebels in Ukraine's east.