Russia has resumed a military buildup near Ukraine, NATO Secretary General Anders Fogh Rasmussen said Thursday, calling it "a very regrettable step backward."
"I can confirm that we now see a new Russian military buildup — at least a few thousand more Russian troops deployed to the Ukrainian border — and we see troop maneuvers in the neighborhood of Ukraine," Rasmussen said in London.
"If they're deployed to seal the border and stop the flow of weapons and fighters that would be a positive step. But that's not what we're seeing."
The Russian Defense Ministry refused to comment on Rasmussen's claim.
Russian officials responded angrily to previous NATO claims of a massive Russian military presence near the 2,000-kilometer border, calling them overblown and insisting that the troops there were stationed quite far from the border and were involved in regular training.
Still, President Vladimir Putin last month ordered troops in the areas near the border to return to their permanent bases elsewhere in Russia. NATO said in late May that the bulk of an estimated 40,000 troops had pulled back.
The new military deployments, if true, would come at a delicate time. On Wednesday, Ukrainian President Petro Poroshenko offered a unilateral cease-fire to jump-start his plan to end fighting in the country's east, where government forces have struggled to suppress a pro-Russian insurgency for two months.
Next week, foreign ministers and leaders of the European Union are scheduled to hold meetings at which relations with Ukraine and Russia will be key discussion topics, including whether Russia's actions toward Ukraine warrant imposing tougher economic sanctions.
Rasmussen said Russia appears to be using its military to intimidate Ukraine further.
"I consider this a very regrettable step backwards and it seems that Russia keeps the option to intervene further," Rasmussen said. "So the international community would have to respond firmly if Russia were to intervene further. That would imply deeper sanctions which would have a negative impact on Russia."
In his speech, Rasmussen said the US-led NATO alliance is at a turning point.
"The world that we helped to build after the end of the Cold War is being challenged in different ways and from different directions," he told his audience at Chatham House.
"To our east, Russia's aggression against Ukraine is an attempt to rewrite international rules and recreate a sphere of influence. At the same time, to our south, we see states or extreme groups using violence to assert their power. And overall, we see threats old and new, from piracy to terrorism to cyber-attacks."