GENEVA (Reuters) - The U.N. human rights office said on Friday it has received "credible reports" of several cases of Russian forces using cluster munitions in populated areas in Ukraine, adding that indiscriminate use of such weapons might amount to war crimes.

       "Due to their wide area effects, the use of cluster munitions in populated areas is incompatible with the international humanitarian law principles governing the conduct of hostilities," spokesperson Liz Throssell told Geneva-based journalists.

       "We remind the Russian authorities that directing attacks against civilians and civilian objects, as well as so-called area bombardment in towns and villages and other forms of indiscriminate attacks, are prohibited under international law and may amount to war crimes."

       Asked about a potential change in Facebook policy that would allow some users to call for violence against Russians and Russian soldiers, Throssell called it "concerning" and said her office would raise it with the company.

       (Reporting by Emma Farge; Editing by Alex Richardson)