Russia has begun pulling most of its roughly 40,000 troops away from its border with Ukraine, a move that U.S. and NATO officials called encouraging.
Defense Secretary Chuck Hagel said the movement of soldiers and equipment was promising but added that all of the Russian troops shifted to the area since March needed to be withdrawn.
Russian President Vladimir Putin announced the pullback Monday, saying the troops had concluded what he labeled military exercises. But with fighting in Ukraine between pro-Russia separatists and government troops continuing, U.S. officials are holding off on declaring the crisis over.
A U.S. Defense official said there were indications from satellite imagery and other intelligence sources that Russian forces were packing up and leaving in large numbers.
In some places, the troops were preparing to load armored vehicles onto rail cars, an apparent sign they were being transported back to bases away from the border, the official said. Some areas where the troops had been camped out for weeks are now deserted, he added.
"We're seeing movements or preparations to move in various stages, but there's still a capable force that remains," said the official, who requested anonymity while discussing intelligence.
Seven Russian battalions — several thousands troops — remain in place near the border, officials said.
"However, there are still quite a number of Russian troops ready to take action if a political decision is taken," he said. "So we continue to call on Russia to stop supporting armed pro-Russia gangs and seal the border so that we don't see arms and fighters crossing into Ukraine."
Rasmussen said ambassadors from NATO countries and Russia would meet in Brussels on Monday to discuss Ukraine, the first face-to-face session since NATO froze relations with Moscow after Russia annexed Ukraine's Crimean peninsula in March.
"We are disturbed by the ongoing violence in eastern Ukraine," White House Press Secretary Jay Carney said, noting that the helicopter attack "indicates separatists continue to have access to advanced weaponry and other assistance from the outside."