Eleven more Ukrainian soldiers were killed in the space of 24 hours while hundreds of bodies of rebels were found in shallow graves in a former separatist stronghold, the army said.
Fighting has escalated sharply since Friday with the downing of a Ukrainian military transport plane and the deaths of civilians in air and artillery attacks on residential areas on both sides of the border, which Russia and Ukraine have blamed on each other.
Accusations of direct Russian involvement in the three and a half month conflict, in which hundreds have died, is being pushed hard by Ukraine to persuade the United States and its European allies to impose tougher sanctions on Russia.
In telephone conversations with German Chancellor Angela Merkel and Herman Van Rompuy, President of the European Council, late on Tuesday, President Petro Poroshenko again set out evidence of fighters crossing into Ukraine from Russia with heavy military equipment, his website said.
Prime Minister Arseny Yatseniuk leveled harsh criticism at Russian President Vladimir Putin.
"Everything which is happening in Ukraine has been planned by Russia since 2004. Putin has a clear plan and that is to destroy Ukraine and establish his influence over post-Soviet space," he said in a public speech carried by his website.
Andriy Lysenko, a spokesman for Ukraine's defense and security council, told journalists that separatists had kept up attacks overnight on government positions along the border.
Government troops had been ambushed by separatists at Izvarino on the border and there had been early morning clashes near the border settlement of Stepanivka when separatists tried to break out of encirclement by the army.
"There was tank and mortar fire and from rockets on positions of the (Ukrainian) 'anti-terrorist operation'," Lysenko said.
The number of casalties was being established.
Lysenko renewed charges that Russia was building up its forces near the border.
But he said, after being temporarily grounded on Tuesday after the downing of an An-26 transporter, Ukrainian warplanes had been given the go-ahead to resume flights over the east.
"They...are already supporting our ground forces in those regions where the toughest clashes are going on," he said.
The new casualty figures would appear to bring to nearly 270 the number of Ukrainian servicemen killed since the government launched an "anti-terrorist" operation in April to crush the rebels.
Hundreds of civilians and rebels have also been killed.
Lysenko said that in Slavyansk, a former rebel stronghold re-taken by government forces, "hundreds of bodies of...(rebel) fighters" had been found under a light covering of earth.
"Some of these graves have been mined by the terrorists."
Violence erupted in Ukraine's Russian-speaking east in April after a pro-Europe revolt in Kiev that ousted a Moscow-backed president and led to Russia's annexation of Crimea, causing the biggest Russia-West crisis since the Cold War.
A 'contact group' of officials from Ukraine, Russia and the Organization for Security and Co-operation in Europe (OSCE) said it tried on Tuesday to re-activate peace talks - but a planned video link-up with separatists never materialized.
"In the opinion of the contact group, this indicates a lack of willingness on the side of separatists to engage in substantive talks on a mutually agreed ceasefire," it said in a statement.
It urged separatists to return to talks immediately.
Ukraine has accused Russia repeatedly of turning a blind eye to Russian fighters crossing the long, shared border to team up with rebels, often with Russian weapons and military equipment.
Moscow denies this.
In an increasing war of words and mutual recrimination, Moscow accused Ukrainian forces of firing a shell across the border last Sunday, killing a Russian man in a Russian border town.
Kiev denies its forces were to blame and says the shell was the work of rebels out to discredit government forces.
Eleven people were said by local health authorities to have been killed in an air strike at the Ukrainian town of Snizhne, 20 km (12 miles) from the border.
Kiev denied rebel charges that the strike had been carried out by a Ukrainian warplane and said it was the work of an "unknown" plane - apparently an accusation against Russia, since the rebels have not used aircraft in the conflict.
Kiev has also said that a missile that brought down the An-26 military transporter was probably fired from Russian territory.
Kiev says it has found four survivors from the eight people who were on board, that two others are being held by rebels, and that it does not know the fate of the remaining two.
Since losing Slavyansk, rebels has been pushed back to the main industrial city of Donetsk though they also remain in control of the border town of Luhansk.
Donetsk, which had a pre-conflict population of about 900,000 people, has been steadily emptying as thousands, fearing a government offensive, have fled.
Once a bustling city, there are fewer and fewer people out on the streets and few cars.
Most of the banks, shops and bars have closed, cash-dispensers have been switched off and street traffic lights no longer work.
Lyudmila, 35, who was leaving Donetsk, said: "I have been forced to gather up my things and get away from this lawless, genocide, which is happening. I am simply saving my children. My husband is staying behind. I have left my home and my work. I am leaving to save my children."
Source: Google News