The White House says President Barack Obama spoke by phone Tuesday with Russian President Vladimir Putin, urging the Russian leader to back a peace deal for Ukraine set for review Wednesday at a four-nation crisis summit in Belarus.
Obama's call, described in a White House statement, came as Ukraine reported that rocket strikes in the eastern city of Kramatorsk killed at least 12 people and wounded more than 80 others.
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The Ukraine Foreign Ministry blamed pro-Russian separatists for the latest violence, calling the rocket barrage "a deliberate massacre of the city's civilians."
The White House statement said Obama "underscored "the rising human toll" of the fighting, as well as the importance of Putin "seizing the opportunity" to back peace proposals when he meets Wednesday with the leaders of France, Germany and Ukraine in Minsk.
Kyiv and a host of Western governments accuse Moscow of backing the nearly yearlong rebellion in Ukraine's east with troops, strategic planning and military hardware.
The president emphasized the importance of reaching and implementing a negotiated settlement. He said, however, if Russia continues its aggressive actions in Ukraine - including sending troops, weapons, and financing the separatists - the costs for Russia will rise.
Meanwhile, rocket strikes in the eastern Ukrainian city of Kramatorsk on Tuesday hit the headquarters of the military's operation against Russia-backed separatists and residential areas ahead of a key summit aimed at ending the conflict.
Ukrainian National Security and Defense Council Secretary Oleksandr Turchynov said that according to "preliminary data," the rebel rocket attacks on Kramatorsk killed 15 people and injured another 63, including five children.
The Donetsk regional administration reported that 32 of those injured were servicemen at an airfield struck by a rocket.
President Petro Poroshenko told Ukraine's parliament that rebels launched two salvos targeting Kramatorsk: one trageting the headquarters of the "anti-terrorist operation" in eastern Ukraine, while the second hit residential areas of the city.
Ukrainian presidential adviser Yuri Biryukov wrote on Facebook earlier Tuesday that two servicemen were killed and nine others injured at the military headquarters.
VOA's Daniel Schearf, who is reporting from Kramatorsk, was nearby when the rocket attack occurred.
He tweeted shortly after the incident: "I'm in #Kramatorsk. Was 2km down the road when Grads struck. Crazy loud. People cowering in fear. They hoped the war had finished here."
Schearf also saw an unexploded rocket in a residential area of Kramatorsk.
Kramatorsk is more than 50 kilometers (30 miles) away from the nearest fighting in Donetsk.
at rocket strike in . Fragments lodged at residential storage. 1 victim's body nearby.
Prior to the attack, Kyiv military spokesmen said at least seven Ukrainian soldiers had been killed in the last 24 hours of fighting in the east of the country.
At the front in Vuhlehirsk, a small town captured by rebels last week, volleys of artillery crashed in both directions. The rebels are pushing to encircle government forces holding out in nearby Debaltseve, a strategic rail hub that pro-Russian separatists have been trying to pry from Kyiv forces.
Earlier Tuesday, pro-government forces had reported making gains.
The Azov Battalion, a pro-Kyiv volunteer paramilitary group, said via Facebook it had captured several villages during an offensive toward Novoazovsk, a town on the border with Russia.
Kyiv also announced on Tuesday that its forces had launched a counteroffensive for the coastal town of Mariupol, the biggest city in the rebellious provinces still in government hands. Details of that counter-offensive could not be confirmed.
The upsurge in violence comes as representatives of Russia, Ukraine, Germany and France are due to meet in the capital of Belarus on Wednesday to try to come up with a peace deal to end the 10-month conflict that has claimed at least 5,400 lives.
Decrying the rocket attack on Kramatorsk, Poroshenko said the next few days could be vital to Ukraine's and Europe's future.
“Maybe these few days would be crucial, not, most importantly in our history, but in the history of Europe of the 21st century,” he said.