LONDON (Reuters) - Britain said on Saturday that Russia's proposed ceasefire in the Ukrainian city of Mariupol was probably an attempt to deflect international condemnation while giving itself a chance to reset its forces for a renewed offensive.

       "By accusing Ukraine of breaking the agreement, Russia is likely seeking to shift responsibility for current and future civilian casualties in the city," the British defence ministry said in an intelligence update.

       Russia said earlier it had opened humanitarian corridors near the besieged cities of Mariupol and Volnovakha. Russia's defence ministry accused Ukrainian "nationalists" of preventing civilians from leaving, RIA news agency reported. But Mariupol's city council said Russia was not observing the ceasefire.

       (Writing by William Schomberg; Editing by Frances Kerry)