A French diplomatic source says a demilitarized zone between Ukraine and Russia is essential for a cease-fire but is a sticking point in a new international push for peace.

The official said Monday night that the other main difficulties are how to police the Ukrainian-Russian border, and the future status of regions in eastern Ukraine now under control of pro-Russian rebels.

Despite "imperfections" in a new French-German peace plan and some skepticism from Russian President Vladimir Putin, the French official expressed confidence that four-way peace talks will go ahead as planned Wednesday in Minsk.

The official spoke anonymously because of the sensitiveness of the on-going negotiations. Few details have been released of the new plan and ensuing discussions.