A UN human rights report said the surging toll was caused by the fighting moving into densely populated areas of the country.
The number of people who have fled their homes has reached at least 430,000, according to UN figures, with almost half heading to Russia.
It said at least 2,593 people including at least 23 children, have been killed in Ukraine between mid-April when the conflict erupted and August 27, with another 5,956 injured.
In a grim sign of the growing human cost of the fighting, the United Nations said the number of people killed each day has risen to an average 36.
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"Especially concerning is the alleged increased participation of foreign fighters in the hostilities, which further fuels the conflict," Simonovic said, though he cautioned that rights monitors were not in a position to provide military analysis.
"Armed groups continued to commit killings, abductions, physical and psychological torture," the report said.
In a damning statement issued today, Human Rights Watch also said rebels were committing "horrendous crimes" against those in their custody.
Captives told the organisation of stabbings, cigarette burns and mock executions, and HRW said at least six were used as hostages for ransom or exchange.