The leaders of France, Germany, Russia and Ukraine say the truce between government forces and pro-Russian insurgents in east Ukraine has been "generally respected since September 1".
In a phone conversation that lasted for 90 minutes, Francois Hollande, Angela Merkel, Vladimir Putin and Petro Poroshenko said everything must be done "to make this situation last and allow the OSCE special observer mission to have permanent, safe and unrestricted access" to the affected areas, a statement from the French presidency said.
It also said that the four leaders would hold a summit in Paris in "early October" to discuss the peace efforts in Ukraine.
Foreign ministers from the four countries will first meet in Berlin on Saturday (local time) to come up with "concrete proposals", the statement from the Elysee Palace said.
Ukraine's army said earlier on Wednesday that a new nine-day truce with pro-Russian rebels was holding despite small skirmishes that claimed the lives of two government soldiers this week.
The bitter foes agreed on September 1 to more strictly respect an often-broken February armistice.
The United Nations estimates that the 17-month conflict on the European Union's eastern frontier has claimed the lives of nearly 8000 people and injured almost 18,000 - most of them civilians.
Russia denies accusations by Ukraine and its allies of orchestrating and supporting the conflict to try to prevent Ukraine from joining the European Union and possibly NATO at a later date.