NATO Secretary-General Anders Fogh Rasmussen said on Thursday that at least a few thousand more Russian troops were now at Ukraine's eastern border, a build-up he called a regrettable step backwards.
"We now see a new Russian military build-up around the Ukrainian border. At least a few thousand more Russian troops are now deployed," Rasmussen said in London.
"I consider this a very regrettable step backwards. It seems Russia keeps the option open to intervene further in Ukraine," he said. "The international community would have to respond in a firm manner if Russia were to intervene further in Ukraine."
He added: "That would imply deeper ... economic sanctions against Russia which would have a very damaging effect on the Russian economy."