Unlike Crimea, which was quickly swept by Russian forces who met no resistance from Ukrainian troops, an invasion into the east would likely trigger fighting that could quickly erode public support at home for Putin's expansionist drive.
And from an economic viewpoint, taking control of a huge swath of territory in the east, which accounts for nearly a quarter of Ukraine's population of 46 million and the bulk of its industrial wealth, would require huge investments that could be beyond Russia's capacity.
Fyodor Lukyanov, chairman of the Council for Foreign and Defense Policy, an association of leading political experts, said making Ukraine a federation would allow the Kremlin to retain control by rendering the country too amorphous to join any Western blocs.
Lukyanov warned in remarks on the Slon.ru online newspaper that even though Putin may prefer to achieve his goals by non-military means, bloodshed in the east would likely trigger a Russian invasion.
"If Ukrainian special forces and law enforcement agencies move to violently suppress the demonstrations ... Russia will have to pull into that," he said.
About 40,000 Russian troops deployed alongside the Ukrainian border have underlined Russia's readiness to act. Moscow rejected Western demands to pull them back, saying that it has the right to station the military wherever it wants on its territory.
An assortment of satellite images provided to The Associated Press by NATO headquarters showed dozens of Russian tanks and other armored vehicles, combat jets and helicopter gunships stationed in areas along the Ukrainian border. But the photographs were not enough to judge the full scope of the deployment.
NATO's top military commander in Europe, U.S. Air Force Gen. Philip Breedlove, told the AP that countermoves to the Russian military threat against Ukraine could include sending American troops to alliance nations in Eastern Europe that feel at risk.
"Essentially what we are looking at is a package of land, air and maritime measures that would build assurance for our easternmost allies," he said.
In Luhansk, masked protesters warned the government that any attempt to storm the headquarters of Ukraine's Security Service they occupied would be a "welcome to hell." Hundreds of supporters camped outside the building overnight, shouting "Putin! Putin!" They erected high barricades along a thoroughfare running in front of the building.
In another eastern city of Donetsk, where protesters were still occupying another government building, regional governor Serhiy Taruta met with key activists to negotiate a peaceful solution. Both he and the activists sounded optimistic after the talks.
"I'm sure that we will solve this peacefully," Taruta said. "Any forceful measures will only aggravate the situation."
Associated Press writers John-Thor Dahlburg in Paris, Maria Danilova in Kiev, Laura Mills in Moscow and Monika Scislowska in Warsaw, Poland, contributed to this report.
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