Russian President Vladimir Putin and his Ukrainian counterpart Petro Poroshenko will meet next week in Belarus with EU representatives, the Kremlin said Tuesday.

Meanwhile Poroshenko's office said leaders would use the August 26 meeting to discuss ways to stabilize the situation in eastern Ukraine, energy security and Kiev's new trade deal with the European Union, which Russia has opposed.

Ukraine said Tuesday that fighting had erupted in the heart of the major rebel stronghold of Lugansk as government forces pressed on with a punishing offensive to win back the war-torn east.

Kiev's military claimed street battles with insurgents were raging in the city center after one outlying district was "liberated".

German Chancellor Angela Merkel will visit Ukraine Saturday amid international tensions over fighting in the east of the country, Ukraine's foreign minister Pavlo Klimkin said.

"The visit is planned for the day before Ukraine's national holiday" on August 24, Klimkin told journalists, adding that the visit by the German leader would be "very interesting".

Anastasiya clutches two empty five-liter bottles as she joins a queue of Donetsk residents buying water in the rebel-held east Ukrainian city where supplies were abruptly cut off by shelling damage.

"We hope it's not for long," says the 32-year-old as she waits for her turn to fill the bottles with mineral water sold at one of many stalls around the city for .75 hryvnia ($.06) a liter.

Ukraine on Monday accused pro-Russian rebels of killing dozens of civilians fleeing the war-torn east, as crisis talks between Kiev and Moscow failed to halt months of bloodshed.

Kiev's military said insurgents using Russia-supplied weapons shelled adults and children in a convoy with white flags on a road from the restive city of Lugansk, leading to "dozens of dead".

Russian Foreign Minister Sergei Lavrov said Monday that no progress was reached on establishing a ceasefire or starting peace negotiations over Ukraine during crisis talks in Berlin with his counterpart from Kiev.

"One place where we cannot report positive results is in, first and foremost, establishing a ceasefire and (starting) a political process," Lavrov told a Berlin press conference that was carried live by Russian state television.

Ukrainian forces battled into a key rebel bastion as fresh claims that rocket launchers had crossed over from Russia stoked tensions Sunday ahead of talks between Kiev and Moscow's top diplomats.

The foreign ministers of Russia, Ukraine, Germany and France gathered for a snap meeting in Berlin, after Western leaders had scrambled frantically to contain the crisis in Ukraine.

European leaders on Saturday scrambled to ease tensions over claims that Ukrainian forces had destroyed Russian military vehicles as Kiev inched closer to allowing a mammoth aid convoy from Moscow over its border.

While a diplomatic push sought to defuse the rhetoric, deadly shelling pummeled besieged pro-Russian rebel strongholds and a top separatist leader claimed he had received a fresh injection of troops trained "on Russian territory."

After shelling ended overnight in a suburb of Donetsk in eastern Ukraine, Konstantin Luchkin emerged from his cellar to find his beehives smashed, the facade of his house blown off and his car burnt and puckered with shrapnel holes.

"Call that a targeted strike?" asked the coal miner Saturday, holding his four-year-old daughter Masha as she played on a tablet computer.

Diplomatic efforts to douse an international firestorm over claims that Ukraine's forces destroyed Russian military vehicles ratcheted up Saturday as Moscow demanded that Kiev allow its mammoth aid convoy to cross the volatile border.

Moscow and Kiev's foreign ministers prepared for an urgent meeting with their French and German counterparts Sunday after the United States blasted Russia for its "extremely dangerous" escalation of the crisis in Ukraine.