This satellite image provided by the Supreme Headquarters Allied Powers Europe shows what Nato identifies as Russian self-propelled artillery in Ukraine
Nato released satellite images it said showed columns of Russian armed forces inside Ukrainian territory.
Nato Brigadier General Niko Tak said more than 1,000 Russian troops were operating inside Ukraine, both supporting the separatists and fighting on their side.
He told the BBC there had been a "significant escalation in the level and sophistication of Russia's military interference in Ukraine" over the past two weeks.
The Ukrainian and Russian ambassadors addressed an emergency session at the UN
At Thursday's emergency session of the UN Security Council, US Ambassador Samantha Power said Russia had "outright lied" about its role.
Her Russian counterpart Vitaly Churkin did not respond directly to Western accusations, but said: "There are Russian volunteers in eastern parts of Ukraine. No-one is hiding that".
He hit out at the Ukrainian government, accusing it of "waging war against its own people".
He also questioned the presence of Western advisers in Ukraine and asked where Ukrainian troops were getting their weapons from.
War in eastern Ukraine: The human cost
At least 2,119 people had been killed and 5,043 wounded since mid-April, a UN report on 7 August said
951 civilians have been killed in Donetsk region alone, the official regional authorities said on 20 August
Official casualty counts only record certified deaths while in some particularly dangerous parts of the war zone, such as Luhansk region, victims are said to have been buried informally, for instance in gardens
Rebels (and some military sources) accuse the government of concealing the true numbers of soldiers killed
155,800 people have fled elsewhere in Ukraine while at least 188,000 have gone to Russia.
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