U.S. Ambassador to Ukraine Geoffrey Pyatt has described Deputy Prosecutor General Vitaliy Kasko's resignation as a "blow to Ukraine's reform progress."

"Kasko was a champion for change w/in the Prosecutor General's Office. His resignation is a blow to #Ukraine's reform progress," Pyatt wrote on Twitter on Monday.

The U.S. Ambassador stresses Ukraine deserves a clean judiciary and will require top-to-bottom rule of law reform to address pervasive corruption and cronyism.

"[The] United States will continue to assist those in prosecutorial domain committed to operating under new rules," Pyatt tweeted.

As was reported, Kasko announced his resignation on Monday, February 15.