"In Donetsk City, a SMM UAV recently spotted a “huge arsenal,” including 23 T-72 tanks, multiple launch rocket systems, self-propelled artillery systems, and up to 60 military trucks," Baer said in a statement posted on the website of the U.S. Mission to the OSCE on July 21.

Read alsoFormer SBU chief says Russian proxies have more tanks than Germany and U.K. together"Given the proximity to the line of contact, each of these weapons constitutes a violation of the terms of the Minsk agreements," he said.

He also insists that Russian proxies granted OSCE SMM monitors access to the occupied areas in Donbas, saying that for the third week in a row, the SMM was denied access to proposed sites for disengagement.

"Combined Russian-separatist forces grant the SMM access only sporadically and only when, as Deputy Chief Monitor [Alexander] Hug points out, there is nothing for monitors to see," he added.

Read alsoAlexander Hug: "We note a sustained supply of ammunition in areas not controlled by Gov't"He also cited Hug saying that "without monitoring, there can be no verification; without verification, no trust; without trust, no lasting end to the violence."

"That's why unfettered, unrestricted, and unlimited SMM access, particularly to border areas, is so important. Russian-separatist forces are responsible for the majority of the restrictions placed upon the SMM," Baer said.

He called on Russia and the separatists it supports to stop their systematic efforts to restrict SMM access and to blind the SMM by jamming and shooting down its UAVs.

"Real and sustained SMM access to the border would be a signal to Ukraine and the international community that the combined Russian-separatist forces are ready to resolve the conflict using peaceful, diplomatic means," he added.