The Ukrainian army smashed rebel checkpoints outside the city of Slaviansk as it launched an operation to take back government buildings being held by pro-Russian separatists. The rebels shot down 2 helicopters and heavily damaged a third in what's being described as the most intensive effort yet to win back the city.

Ominous warnings from Moscow made it clear that it may just be a matter of time before Russian troops cross the border to intervene.

Reuters:

Ukrainian officials said troops overran rebel checkpoints around the city in an operation launched before dawn and it was now "tightly encircled". They pointed to the heavy fire that hit the helicopters as proof of the presence of Russian forces, despite repeated denials from Moscow that it has troops on the ground or is controlling the uprising.

Putin's spokesman heaped blame on the Ukrainian government, which took power two months ago after pro-Western protests forced the Kremlin-backed elected president to flee to Russia.

Noting that Putin had warned before that any "punitive operation" would be a "criminal act", Dmitry Peskov told Russian news agencies that this was what had now happened at Slaviansk, where separatists seeking independence or annexation by Moscow are holding seven foreign European military observers.

Saying Putin had sent an envoy, Vladimir Lukin, to southeast Ukraine to negotiate their release, Peskov said that Lukin had not been heard from since the Ukrainian operation began.

"While Russia is making efforts to de-escalate and settle the conflict, the Kiev regime has turned to firing on civilian towns with military aircraft and has begun a punitive operation, effectively destroying the last hope of survival for the Geneva accord," he said, referring to a deal on April 17 signed by Russia, Ukraine, the United States and the European Union.

Under that agreement, separatists were supposed to lay down their arms and vacate the public buildings they have seized in about a dozen towns they have seized across the Russian-speaking east. Since then, however, they have tightened their grip.

Gunmen in Slaviansk said they were holding their ground against Ukrainian forces that have struggled to assert control over the past month. Some threw up new barricades of felled trees. And dozens of civilians blocked a road against a unit of Ukrainian armored vehicles, telling troops to go home.

Reuters journalists in the city heard shooting from shortly after 4 a.m. (9 p.m. EDT Thursday) and saw a military helicopter open fire. Towards midday, the city was quiet, shops were shut but rebel gunmen appeared to be still in tight control of the streets. Ukrainian troops were at a halt in the suburbs.

It's hard to see how the May 25 vote can be considered legitimate if large swaths of Ukrainian terroritory is not under the government's control. Any vote tallies coming from the occupied towns and cities would have to be suspect given the circumstances.

But not holding the vote would probably induce Russian intervention - if they don't intervene before then. Putin has the Ukrainian government well and truly trapped. If they move too aggressively to win back towns and cities that are currently occupied, they risk a move from Moscow. If they do nothing, the rebels continue to expand their holdings. And the ineffective course they are pursuing now - hesitant, half hearted efforts to regain control - may end up giving Putin an excuse to invade anyway and aren't slowing down the separatists at all.

Meanwhile, Obama thinks the NBA did the right thing in banning Donald Sterling and the Benghazi emails weren't about Benghazi at all.