BUCHA, Ukraine: Russian forces on Tuesday were preparing for an offensive in Ukraine's southeast, the Ukrainian military said, as President Volodymyr Zelenskyy prepares to talk to the United Nations Security Council amid outrage over increasing evidence that Moscow's soldiers deliberately killed civilians.

Russian President Vladimir Putin's government is pouring soldiers into Ukraine's east to gain control of the industrial heartland known as the Donbas. That follows a Russian withdrawal from towns around the capital Kyiv, which led to the discovery of corpses, prompting accusations of war crimes and demands for tougher sanctions on Moscow.