Ukraine's political and civic leaders have held a first round of talks to end the crisis in the country - but separatists were not represented.
Correspondents said talks were heated although there was a consensus that Ukraine must remain a united state.
The interim president said Kiev was prepared to listen to rebels, but said they must lay down their arms first.
The meeting took place one day after pro-Russian militants attacked government forces, killing seven.
The seven soldiers were killed when rebels ambushed an armoured personnel carrier near the town of Kramatorsk in Donetsk region. One rebel is said to have also died.
Donetsk and the neighbouring region of Luhansk have declared themselves separate from Ukraine after referendums deemed illegal by Kiev, the US and EU, but Russia said it "respected" the votes.
The struggle for control of Ukraine, where the elected pro-Moscow president was deposed in February, has set Russia at loggerheads with Western powers.
In other developments:
•The interim government said the annexation of Crimea by Russia earlier this year cost Ukraine at least 1tn hryvnya (£49.3bn; $83bn; 60.5bn euros)
•Russian Deputy Prime Minister Dmitry Rogozin said he would not support a US and European proposal to extend the international space station beyond 2020 but US space agency Nasa, which relies on Russia to ferry astronauts to the station, said it had not yet received any official notification on changes in space co-operation
•Russian troops moved to begin constructing a pipeline to supply Crimea with water weeks after Ukraine cut off supplies
The talks in Kiev are part of an initiative put forward by the Organisation for Security and Co-operation in Europe (OSCE) and veteran German diplomat Wolfgang Ischinger was brought in to moderate.
Those present included interim President Olexander Turchynov, Prime Minister Arseniy Yatsenyuk and several former Ukrainian leaders.
'A real war'
The OSCE said Russian President Vladimir Putin supported its initiative.
But Russian Foreign Minister Sergei Lavrov questioned whether presidential elections on May 25 should take place because Ukraine was on "the brink of civil war."
In an interview with Bloomberg Television, Mr Lavrov said in the east and south of Ukraine "there is a war, a real war."
He also called for the inclusion of rebels in the crisis talks, saying they would only succeed if there was "equal participation of all regions".
Ukrainian and Western officials have insisted that the elections go ahead and accused Moscow of being behind the unrest.
Ukraine's interim President Oleksandr Turchynov said that Kiev was ready to negotiate with pro-Russians but that the rebels must give up their arms first.
"We will not yield to blackmail," he said. "We are ready to listen to the people of the east but they must not shoot, loot or occupy government buildings."
The interim government has refused to talk to the separatists, who apparently lack a single leader and agreed goals.
"The government in Kiev does not want to listen to the people of Donetsk," Denis Patkovski, a pro-Russian rebel in Sloviansk, Donetsk region, told AP. "They just come here with their guns."
'Fighting means poverty'
But one of Ukraine's wealthiest businessmen has called on all sides to find a solution to the crisis, saying the majority in eastern Ukraine support unity.
Rinat Akhmetov, who is seen by many as one of the most influential people in the south-east of the country, said the Donbass region of Donetsk and Luhansk could only benefit from a united Ukraine.
"Neither Russia, nor Donbass will win from this. We will face huge sanctions, we will not be able either to sell or to produce. This means the suspension of production, this means unemployment, this means poverty," he said.
Armed separatists continue to occupy key government buildings in the east while Russia denies fomenting the unrest just over its border.
Nato believes some 40,000 Russian troops are deployed near Ukraine's border, although Moscow says they have been pulled back.
Russia annexed Ukraine's southern autonomous republic of Crimea in March following a controversial referendum and the interim government fears a similar outcome in Donetsk, Luhansk and parts of the south.
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