(Adds NATO military officer comments)
LONDON, June 19 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.
After Moscow annexed Ukraine's Crimea region in March, NATO said Russia had massed some 40,000 troops near the Ukrainian border. It withdrew the vast majority of them until just one unit and 1,000 troops remained about a week ago.
"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."
In last few days, NATO has seen evidence of a few mechanised units, consisting of a few thousand Russian troops, conducting new troop movements close to the Ukrainian border, a NATO military officer said.
"These troops don't appear to be engaged in 'border patrol' duties. Rather they appear to be concentrating in staging areas and preparing and awaiting future orders," the officer said. (Reporting by William James and Jack Stubbs; Additional reporting by Adrian Croft in Brussels; writing by Guy Faulconbridge; Editing by Tom Heneghan)