By Guy Faulconbridge and Vladimir Soldatkin 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.

But Putin, speaking at an international economic forum in St Petersburg, said Russia had never doubted Ukraine's right to sovereignty, but noted that when Ukraine declared its independence in 1991 it was as a "neutral state".