MOSCOW—Ukrainian President Petro Poroshenko said Monday that he would unveil a detailed peace plan in the coming days to end fighting with separatists in the east and that a cease-fire could be achieved as early as this week, despite continuing clashes.

Mr. Poroshenko laid out the broad outlines of the peace plan during his inauguration this month, saying he was willing to negotiate and offer an amnesty to fighters who didn't have "blood on their hands." The remarks appeared to backtrack from his call following...