"We are docking those difficult provisions that should sort of determine the form and fate of the future document," Ushakov told Russian journalist Pavel Zarubin.
At the same time, he said the exact wording and coordination with legal specialists are yet ahead.
"[Some issues] require the US to make serious, I would say, radical changes to their papers, which are constantly in motion, constantly being prepared," Ushakov said.
He stressed that the negotiation process is aimed at achieving not a ceasefire but a "long-term settlement."