ST PETERSBURG, Russia (Reuters) -Russian President Vladimir Putin quipped on Friday that in his view the whole of Ukraine was "ours" and cautioned that advancing Russian forces could take the Ukrainian city of Sumy as part of a bid to carve out a buffer zone along the border.

Moscow has hoped to mend relations with the United States while continuing to wage war in Ukraine, but Mr. Putin’s insistence on pushing his offensive has come at a diplomatic cost.