Pyatt said those systems included surface to air missiles, artillery, and rockets.
He also accused Russian military units "along the border" of "preparing a large shipment of supplies" to the Russian-backed separatist fighters.
The fresh accusations come as the clock ticks down a midnight ceasefire agreed to on Thursday as part of what aims to be a broader deal to end the nearly year-long conflict that has left more than 5,000 people dead and more than a million displaced.
The images, taken by the commercial satellite image company DigitalGlobe and tweeted out by Pyatt, are of an area near the village of Debaltseve.
Debaltseve has been the scene of heavy fighting in recent days, as both sides battle for control of its railway infrastructure.
Pyatt claimed on Twitter that the rebel troops "now have larger fighting force with more tanks armored vehicles, heavy artillery & rocket systems than some NATO and European countries."
It appears both sides are continuing their attacks ahead of the ceasefire.
On Saturday, heavy shelling landed near the center of the rebel-held city of Donetsk. Several people were reportedly injured.