A recent surge in fighting in eastern Ukraine, where Kiev is fighting pro-Russian separatists, and fresh tension in Crimea have raised concern that a fragile ceasefire agreed in Minsk in February 2015 could collapse.

Russian's Vladimir Putin held a phone call with German Chancellor Angela Merkel and French President Francois Hollande on August 23, the Kremlin said. Putin, it said, drew attention to "provocations" by Ukraine in Crimea. Russia annexed Crimea in 2014 after protesters overthrew Kiev's pro-Moscow president.

The Kremlin also said concern was expressed about instability along the line of confrontation in east Ukraine.

A German government spokesman said the three leaders shared concern in their phone call about ceasefire breaches in Ukraine's Donbass region and that Merkel and Hollande had urged Putin to do what he could to calm the situation.

Putin, Merkel and Hollande also discussed the implementation of the Minsk agreements. "There was agreement that progress was urgently needed and that in particular a stabilization of the ceasefire had to be achieved," said the German spokesman.

Ukrainian President Petro Poroshenko said last week he did not rule out introducing martial law and a new wave of military mobilization if the separatist conflict worsened.