This comes five days before the Syrian government and opposition are scheduled to meet in Geneva.
"We are working hard – our foreign minister [Sergei Lavrov] is due to meet with the US secretary of state [John Kerry]. Work is continuing in other areas, but not everything is running smoothly," Dmitry Peskov confessed.
Moscow and Washington have disagreements on what factions in Syria should be considered opposition and who should be designated as terrorists, the spokesman added.
"Profound differences remain as to who should be on the ‘white’ list and who should be put on the ‘black’ list. So the field of work is still very wide," he said.
US State Department spokesman John Kirby vowed Tuesday that differences between the two countries would not prevent continued progress in the process to end the five-year Syrian war.