He wrote this in a series of tweets, Ukrinform reports.
"Here is footage from the OSCE SMM of convoys of trucks illegally entering and exiting Ukraine from Russia. As a reminder, Russia has blocked an effective border monitoring mission and its forces routinely prevent the SMM from visiting the border," Volker said.
He also posted a video taken from the OSCE's unmanned aerial vehicle, which documented the movement of convoys of military vehicles across the border.
In another tweet, the U.S. diplomat drew attention to the Kremlin's true role in the conflict against Ukraine.
"This is yet another way that Russia deliberately prolongs the conflict and sustains the fighting in eastern Ukraine despite international efforts to bring peace for Ukraine," Volker wrote.
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