Russia is again building up forces at the border with Ukraine, NATO Secretary-General Anders Fogh Rasmussen said on Thursday, calling it a "very regrettable step backwards".

Russia is again building up forces at the border with Ukraine, NATO Secretary-General Anders Fogh Rasmussen said on Thursday, calling it a "very regrettable step backwards".

Rasmussen said "at least a few thousand more" Russian troops had been deployed to the border.

"I consider this a very regrettable step backwards. It seems that Russia keeps the option open to intervene further into Ukraine," he told an audience at the Chatham House foreign affairs think-tank in London.