US President Barack Obama has accused Russia of being responsible for the violence in eastern Ukraine.
He said the fighting was not the result of a home-grown indigenous uprising but of “deep Russian involvement”.
Mr Obama said fresh satellite images of Russian forces inside Ukraine made its role “plain for the world to see”.
Russia denies claims by Nato that more than 1,000 Russian troops are fighting with the separatists. It accuses Ukraine of attacking its own people.
Nato is to hold an emergency meeting on Friday to discuss the crisis.
It follows Thursday’s emergency meeting of the United Nations Security Council in New York.
Pro-Russian separatists have recently opened a new front in the conflict, seizing the south-eastern coastal town of Novoazovsk.
It has raised fears that the Kremlin might want to create a land corridor between Russia and Crimea – a territory annexed by Russia from Ukraine in March.
“There is no doubt that this is not a home-grown, indigenous uprising in eastern Ukraine,” Mr Obama told a news conference.
“Russia is responsible for the violence in eastern Ukraine. The violence is encouraged by Russia. The separatists are trained by Russia, they are armed by Russia, they are funded by Russia. Russia has deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine and the new images of Russian forces inside Ukraine make that plain for the world to see.”
However, he again ruled out a military response from the US.
Mr Obama said Russia would incur “more costs and consequences” for its actions in Ukraine. He is due to discuss the crisis with European leaders at a Nato summit in the UK next week.
On Thursday, Ukrainian President Petro Poroshenko held an emergency security council meeting over the “sharp aggravation of the situation”.
“Russian forces have entered Ukraine,” he said in Kiev, urging people to remain calm.
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.
“[Nato has] detected large quantities of advanced weapons, including air defence systems, artillery, tanks, and armoured personnel carriers being transferred to separatist forces in eastern Ukraine,” he said.
“Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favouring the Ukrainian military.”
Russia’s ambassador to the EU, Vladimir Chizhov, told the BBC that Nato “has never produced a single piece of evidence” of Russian troops operating in Ukraine.
He said the only active duty Russian soldiers in Ukraine were the 10 captured this week, who Russia insists had crossed the border by mistake.
At Thursday’s emergency session of the UN Security Council, Russian Ambassador Vitaly Churkin did not deny the Russian presence, saying “there are Russian volunteers in eastern parts of Ukraine. No-one is hiding that”.
But 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.
He said he wanted to “send a message to Washington: Stop interfering in the internal affairs of sovereign states”.
US Ambassador Samantha Power said Russia “has manipulated, it has obfuscated, it has outright lied.”
And UK Ambassador Mark Lyall Grant said: “Now we see irrefutable evidence of regular Russian forces operating inside Ukraine.”
Ukraine confirmed on Thursday that Novoazovsk had been captured by the rebels, whom they described as “Russian troops”.
It said it had withdrawn its armed forces to save lives, and that Ukrainian soldiers were now reinforcing the defences of the strategic port city of Mariupol.
The port has until now been peaceful and cut off from rebel positions.
Pro-Russian fighters have been trying for weeks to break out of an area further north in the Donetsk region where they are almost encircled.
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.