Ukrainian President Volodymyr Zelenskiy, right, and the European Council President Donald Tusk walk in the town of Stanytsia Luhanska during a working trip to eastern Luhansk region, Ukraine, on Sunday, July 7, 2019. (Ukrainian Presidential Press Office via AP)

European Council president Donald Tusk (2nd L) and Ukrainian President Volodymyr Zelensky (C) walk during a working visit to the village of Stanytsia Luhanska, in Luhansk region, eastern Ukraine on July 7, 2019, following the disengagement of forces and weapons in the area. - The OSCE mission on 26 June had observed the beginning of the disengagement process with Ukrainian Armed Forces personnel and members of the armed formations leaving their most forward positions, together with their weapons, and exiting the disengagement area. (Photo by OLEKSANDR RATUSHNIAK / AFP)

European Council president Donald Tusk (L) and Ukrainian President Volodymyr Zelensky (2nd L) walk during a working visit to the village of Stanytsia Luhanska, in Luhansk region, eastern Ukraine on July 7, 2019, following the disengagement of forces and weapons in the area. - The OSCE mission on 26 June had observed the beginning of the disengagement process with Ukrainian Armed Forces personnel and members of the armed formations leaving their most forward positions, together with their weapons, and exiting the disengagement area. (Photo by OLEKSANDR RATUSHNIAK / AFP)

European Council president Donald Tusk (L) looks on as the Ukrainian President Volodymyr Zelensky (C) shake hands with an OSCE employee during a working visit to the village of Stanytsia Luhanska, in Luhansk region, eastern Ukraine on July 7, 2019, following the disengagement of forces and weapons in the area. - The OSCE mission on June 26 had observed the beginning of the disengagement process with Ukrainian Armed Forces personnel and members of the armed formations leaving their most forward positions, together with their weapons, and exiting the disengagement area. (Photo by OLEKSANDR RATUSHNIAK / AFP)

European Council president Donald Tusk (C-L) and Ukrainian President Volodymyr Zelensky (C) look at a map with an OSCE employee during a working visit to the village of Stanytsia Luhanska, in Luhansk region, eastern Ukraine on July 7, 2019, following the disengagement of forces and weapons in the area. - The OSCE mission on June 26 had observed the beginning of the disengagement process with Ukrainian Armed Forces personnel and members of the armed formations leaving their most forward positions, together with their weapons, and exiting the disengagement area. (Photo by OLEKSANDR RATUSHNIAK / AFP)