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".
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".
Mr Lavrov was speaking after the US accused Russia of lying about its involvement in Ukraine and warned of tougher economic sanctions.
Nato has accused Russia of violating its international obligations and has released satellite photos of what it says are Russian troops in Ukraine.
Secretary-General Anders Fogh Rasmussen said: "It is now clear that Russian troops and equipment have illegally crossed the border into eastern and southeastern Ukraine.
Russian arms seized in eastern Ukraine "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".
A top insurgent in the rebel stronghold of Donetsk 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.
A satellite image allegedly showing a Russian military incursion in Ukraine 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.
The rebels control the cities of Donetsk and Luhansk Nato is due to hold an emergency meeting on the crisis, while German Chancellor Angela Merkel has said an EU summit on Saturday would discuss the prospect of further sanctions against Russia.
The tensions also led Poland to prevent a plane carrying Russia's defence minister from travelling through its airspace on Friday, leading to a complaint from Moscow.
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.
A Ukrainian soldier exchanges fire with separatists 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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Mr Poroshenko (right) demanded action, not words, to end the fighting
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Syrian President Bashar al Assad
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US reporter James Foley
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