Russian Foreign Minister Sergey Lavrov has warned that the Ukrainian people were "in essence being pushed into the abyss of fratricidal war", as government forces step up their operation against pro-Russian armed groups in the eastern region.
His statement came as Ukrainian Prime Minister Arseniy Yatseniuk called on Russia to block the border to Ukraine to prevent fighters from entering his country. Yatseniuk said that if Russian influence was eliminated, the crisis in the east of the counry could be ended swiftly.
"The situation in the east is deteriorating. A number of trucks full of live ammunition, full of Russian-trained guerrillas crossed the Russian border into Ukraine," Yatseniuk said.
"We ask Russia and Putin to block the border to Ukraine ...If Russia is out of this game we can handle this situation in a week, but as they are supporting and financing them, and providing them access to Ukrainian territory this creates huge difficulties for us."
Lavrov repeated on Wednesday Russia's calls for an end to the military action following two days of fierce fighting at the Donetsk regional airport that killed an estimated 50 militia fighters, the Interfax news agency reported.
An uneasy calm returned to the streets of Donetsk on Wednesday. Some shops remained closed and streets were quieter than usual, but about 1,000 miners from around the eastern Donbass coalfield staged a demonstration in support of the separatists.
A Ukrainian fighter jet roared overhead, and some gunfire could be heard in the distance, apparently from rebels shooting at the aircraft.
On Monday and Tuesday, government forces killed dozens of separatist fighters in an assault to retake the regional airport, which the rebels had seized the morning after Ukrainians overwhelmingly elected Petro Poroshenko as president.
Pro-Moscow gunmen have declared the city of a million people capital of an independent Donetsk People's Republic.
After the government assault - the first time Kiev has unleashed its full military force against the fighters after weeks of restraint - mortuaries were filled with bodies of rebel gunmen.
The separatists said as many as 50 had died, including a truckload of wounded fighters blasted apart as they were driven away from the battlefield.
The government said it suffered no losses in the operation, which saw its aircraft strafe the airport and paratroops land to reclaim it.
'State of war'
Poroshenko, a billionaire confectionary magnate who became the first Ukrainian since 1991 to win the presidency outright in a single round of voting, repeated his promise to restore government control rapidly over secessionist-held areas.
"We are in a state of war in the east. Crimea is occupied by Russia and there is great instability. We must react," he told Germany's Bild newspaper.
"The anti-terrorist operation has finally begun in earnest. We will no longer permit these terrorists to kidnap and shoot people, occupy buildings or suspend the law. We will put an end to these horrors - a real war is being waged against our country."
His swift offensive has thrown down a challenge to Russian President Vladimir Putin, who made defending Russians in other parts of the former Soviet Union a pillar of his rule since declaring his right to use military force in Ukraine in March.
Moscow has said it was willing to work with Poroshenko but had no plans for him to visit for talks.
It denies accusations by Kiev and Western countries that it is behind the rebellion.
"I have no doubt that Putin could end the fighting using his direct influence," Poroshenko said. "I definitely want to speak with Putin and hold talks to stabilise the situation."
Meanwhile, Chechnya's Moscow-backed leader Ramzan Kadyrov insisted on Wednesday he had not sent any of his troops to help pro-Russia armed groups but said some Chechens may have gone there on their own.