The Russian president, Vladimir Putin, has hit back at accusations that he has effectively invaded Ukraine, accusing Ukrainian forces of behaving like Nazis in the conflict in the east and ominously threatening to take his standoff with the west into the disputed Arctic.
Hours after Barack Obama accused Russia of sending troops into Ukraine and fuelling an upsurge in the separatist war, Putin retorted that the Ukrainian army was the villain of the piece, targeting residential areas of towns and cities like German troops did in the former Soviet Union.
Nato ambassadors emerged from a meeting yesterday morning to accuse Russia of a "blatant violation" of Ukraine's sovereignty.
Nato told Russia to halt its "illegal" military actions in Ukraine after the West accused Moscow of direct involvement in the escalating conflict.
"This is not an isolated action, but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation," Nato chief Anders Fogh Rasmussen said after an emergency meeting of the alliance.
Kiev and the West have accused Russian troops of being behind a lightning counter-offensive that has seen the rebels seize swathes of southeastern territory from government forces, dramatically turning the tide in the four-month conflict.
And in a move certain to anger Kiev's former masters in Moscow, Rasmussen said Nato was not closing the door to Ukraine's possible membership of the alliance after the government said it was taking steps to join.
Putin has repeatedly denied Moscow is fuelling the conflict or having any troops on the ground in the former Soviet state.
And yesterday, he demanded that the Ukrainian government hold "substantial" talks with the separatists who took up arms against Kiev in April, apparently emboldened by Russia's annexation of Crimea the month before.
He called on rebel forces to open a "humanitarian corridor" for the besieged Ukrainian troops. Top rebel leader Alexander Zakharchenko -- who has said the Russian troops in Ukraine were "on holiday" -- told Russian television his men would be willing to let government troops withdraw if they give up their weapons.
Nato on Thursday said Russia had sent at least 1,000 troops to fight alongside the rebels, along with air defence systems, artillery, tanks and armoured vehicles, and had massed 20,000 troops near the border.
The new rebel advance has raised fears that the Kremlin could be seeking to create a land corridor between Russia and Crimea on the Black Black Sea.
The United States and the European Union have already imposed a series of punishing sanctions on Moscow over the worst standoff between Russia and the West since the Cold War.
According to new UN figures, almost 2,600 people have been killed since mid-April in the crisis, and well over 400,000 have fled their homes, many of them to Russia.