President Obama: "Russia is responsible for the violence in eastern Ukraine"
Nato officials are holding an emergency meeting to discuss the crisis in eastern Ukraine, as the West steps up its accusations of direct Russian involvement in the conflict.
On Thursday 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.
The accusations come as pro-Russian rebels gain ground. More than 2,000 people have died in months of conflict.
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 Thursday 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.
Some French nationals have joined the pro-Russian rebels
Shelling has caused widespread damage in the eastern city of Donetsk
This tank was destroyed in fierce fighting at Savur-Mohyla, east of Donetsk
Residents of the strategic city of Mariupol staged an anti-war rally on Thursday
Ukrainian forces had made gains against rebels until the latest counter-offensive
Overnight, Russian President Vladimir Putin called on the rebels to open a "humanitarian corridor" to allow the Ukrainian troops out of encirclement in order to avoid 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.
'Not home-grown'
The reports of Russian troops fighting with rebels prompted renewed Western criticism of Moscow's role in the conflict. Speaking at a news conference in Washington, US 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."
Mr Obama is due to discuss the crisis with European leaders at a Nato summit in the UK next week.
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.
- 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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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.
