Ukraine has begun a second-round of "anti-terror" operations against pro-Russian militias, as Prime Minister Arseniy Yatsenyuk said today that Russia "wants to start world war three." 

Yatsenyuk wants to warn European leaders that the clashes between the two neighbors could spread elsewhere, saying, "Attempts at military conflict in Ukraine will lead to a military conflict in Europe." He added that "it is clear that Russia's goal is to wreck the election in Ukraine, remove the pro-western and pro-Ukrainian government and occupy Ukraine politically as well as military."

Yatsenyuk today vowed to continue counter-terrorism efforts in the east, where pro-Russia separatists have been occupying  government buildings for weeks. Five separatists were reportedly killed in clashes in Sloviansk yesterday.

A high-ranking Ukrainian official told the Wall Street Journal that troops would blockade Sloviansk to keep out weapons and reinforcements today, but work to avoid further casualties: 

Around midday, a top official in Kiev said the government had decided to start "the second phase" of the operation as Slovyansk remained in pro-Russian control. "We've decided to completely blockade the city of Slovyansk to prevent reinforcements from coming in," said Sergei Pashinsky, chief of staff to acting president Oleksandr Turchynov. "The goal is to blockade the terrorists and prevent any civilian casualties."

Pashinsky added that any movement by Russian troops across the border would lead to a Ukrainian military response. Yesterday, Russia ran military exercises close to the Ukrainian border in a less-than-subtle show of force, all but vowing to cross that border if Ukraine doesn't back down.

But efforts to keep activity in Sloviansk peaceful today already appear to have failed. Officials in Kiev reported that earlier today a Ukrainian helicopter was struck by a separatist sniper with an RPG and exploded. The International Business Times report: 

"An army Mi-8 helicopter has exploded at the Kramatorsk aerodrome," the defence ministry in Kiev said. Ukrainska Pravda website reported that two explosions were heard at the military base. Dmitry Tymchuck, head of the centre for military and political research, said the helicopter was hit just before take-off. The pilot managed to jump out of the craft unharmed and there were no reports of injuries.

RT uploaded a video to YouTube showing the dramatic explosion: 

Meanwhile, President Barack Obama is meeting today with a number of European leaders to discuss slapping Russia with additional sanctions. He said"We'll continue to keep some arrows in our quiver in the event we see further deterioration." So far, financial action has only been taken against individuals, but the president said today that industry-wide sanctions would be in place if Russia invades Ukraine. 

Sanctions against Russia have so far had no influence on their behavior, but do seem to be starting to take a toll on the country's economy, or at least the confidence in it. Today, Standard & Poor's lowered Russia's credit rating for the first time in five years, after Moscow's central bank raised interest rates, and yesterday the country's stocks took a hit