NATO is holding an emergency meeting over eastern Ukraine crisis, as the West steps up its accusations of direct Russian involvement in the conflict.
On August 28, NATO released satellite images it said showed Russian forces inside Ukraine and said more than 1,000 troops were operating there.
Russia denies sending troops to eastern Ukraine.
Pro-Russian rebels have gained ground recently. Nearly 2,600 people have been killed since April, the UN says.
Heavy fighting is continuing near the strategic port of Mariupol, on the Azov Sea. Rebel forces are trying to capture the city but Ukrainian government troops are digging in.
On August 28, the separatists seized the nearby town of Novoazovsk.
The advance has raised fears that the Kremlin might seek to create a land corridor between Russia and Crimea – a territory annexed by Russia from Ukraine in March.
Rebels are also reported to have surrounded government soldiers in several places further north, near the city of Donetsk.
Ukraine forces near the town of Ilovaysk say they are cut off and have been urgently asking for supplies and reinforcements.
Overnight, Russian President Vladimir Putin called on the rebels to open a “humanitarian corridor” to allow encircled Ukrainian troops to leave without unnecessary casualties, though he did not specify the location.
Rebel leader Alexander Zakharchenko later told Russian TV that his fighters had agreed to the request, on condition that the Ukrainians hand over heavy weapons and ammunition.
At least 2,593 people had been killed in the conflict between mid-April and August 27, the UN said in its latest report.
Human rights violations like abduction and torture were “committed primarily by the armed groups”, referring to the rebels, it said.
Separately, Human Rights Watch said in a report that the rebels were subjecting civilians to torture, degrading treatment and forced labor.
The reports of Russian troops fighting with rebels prompted renewed Western criticism of Moscow’s role in the conflict.
President Barack Obama blamed Russia for the escalation but stopped short of saying its troops had invaded Ukraine.
“There is no doubt that this is not a home-grown, indigenous uprising in eastern Ukraine,” he said.
“The separatists are trained by Russia, they are armed by Russia, they are funded by Russia.”
Barack Obama is due to discuss the crisis with European leaders at a NATO summit in the UK next week.
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.
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”.
Vitaly Churkin also questioned the presence of Western advisers in Ukraine and asked where Ukrainian troops were getting their weapons from.
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