Russian President Vladimir Putin has said Ukraine's assault on cities in the east of the country is reminiscent of the Nazi siege of Leningrad.
"Small villages and large cities surrounded by the Ukrainian army which is directly hitting residential areas... It sadly reminds me the events of the Second World War, when German fascist... occupants surrounded our cities," he told a youth camp outside Moscow.
Mr Putin urged Kiev to begin "substantive" negotiations with the separatists to achieve peace, adding that Russians and Ukrainians are "practically one people".
"It is necessary to force the Ukrainian authorities to substantively begin these talks - not on technical issues… the talks must be substantive," Mr Putin said.
He also said Russia needed to strengthen its position in the Arctic, economically and militarily.
Mr Lavrov: 'No proof of Russian involvement' Earlier Russian Foreign Minister Sergei Lavrov dismissed claims that its troops are fighting in eastern Ukraine as "conjecture".
"We're hearing various conjectures, not for the first time, but not once have any facts been presented to us," he said.
Mr Lavrov was speaking after the US accused Russia of lying about its involvement in Ukraine and warned of tougher economic sanctions.
Speaking at the White House, President Barack Obama said Russia has been supporting pro-Moscow separatists in eastern Ukraine "for months".
Russian arms seized in eastern Ukraine He ruled out any direct US military intervention but said Russia would incur "more costs and consequences".
"The separatists are backed, trained, armed, financed by Russia. Russia determined that it had to be a little more overt in what it had already been doing, but it's not really a shift," Mr Obama said.
Nato has accused Russia of blatantly violating its international obligations, saying there is clear evidence its troops are moving across the border into Ukraine.
Secretary-General Anders Fogh Rasmussen condemned the action, re-iterating the organisation's unwaivering support for Kiev.
A satellite image allegedly showing a Russian military incursion in Ukraine "Despite Moscow's hollow denials, it is now clear that Russian troops and equipment have illegally crossed the border into eastern and southeastern Ukraine," he said.
"This is not an isolated action but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation."
Kiev said on Friday a call by Mr Putin for separatists to open a 'humanitarian corridor' to allow encircled Ukrainian troops to withdraw was further proof they were "led and controlled directly from the Kremlin".
"I'm calling on insurgents to open a humanitarian corridor for Ukrainian troops who were surrounded in order to avoid senseless deaths," Mr Putin said in a statement.
The rebels control the cities of Donetsk and Luhansk He went on to praise the pro-Russian separatists for "undermining Kiev's military operation which threatened lives of the residents of Donbass and has already led to a colossal death toll among civilians".
A top insurgent in the rebel stronghold of Donetsk promptly reacted to Mr Putin's appeal but said the Ukrainian troops would have to lay down their arms before they were allowed to go.
"We are ready to open humanitarian corridors to the Ukrainian troops who were surrounded with the condition that they surrender heavy weaponry and ammunition so that this weaponry and ammunition will not be used against us in future," Alexander Zakharchenko said on Russia's state Rossiya 24 television.
A Ukrainian soldier exchanges fire with separatists Ten Ukrainian soldiers have been killed and 30 wounded in fighting with pro-Russian separatists in the past 24 hours, Kiev's security and defence council said on Friday.
Fighting has intensified since the rebels - allegedly helped by Russian soldiers - opened a new front just as Ukraine's army had virtually surrounded Donetsk and another separatist stronghold, Luhansk.
The casualty figure was released after the UN revealed a total of 2,593 people have been killed in eastern Ukraine since the fighting erupted in mid-April.
It said the number included civilians as well as Ukrainian and separatist combatants, but not the 298 victims of the MH17 Malaysian Airlines plane crash.
Nato is due to hold an emergency meeting on the crisis later, while German Chancellor Angela Merkel has said an EU summit on Saturday would discuss the prospect of further sanctions against Russia.
Dutch Prime Minister Mark Rutte said on Friday the current sanctions "have not worked and the EU will have to consider additional measures.
Responding to calls by Ukrainian Prime Minister Arseny Yatseniuk for his country to join Nato, Mr Rutte said it would "not contribute to a solution".
Meanwhile, the US fast-food giant McDonald's has revealed a total of 12 branches in Russia had been temporarily closed by the state food safety watchdog over alleged sanitary law breaches.
The company, which has 440 restaurants in the country, also said more than 100 further inspections were being carried out at its outlets.
"We are studying the essence of claims in order to determine the necessary actions for the swift re-opening of restaurants for visitors," the firm said in a statement.
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