MONS, Belgium - A senior NATO official said on Thursday that "well over a thousand" Russian troops were operating inside Ukraine.
"They support separatists, fighting with them and fighting amongst them," the official said on condition of anonymity, adding that the supply of arms by Russia had increased in both "volume and quality".
The official, who was speaking to reporters ahead of a NATO summit next week in Britain, said the situation was made even more worrying because the key route between Donetsk and Novoazovsk, on the Sea of Azov close to the Russian border, had been cut off by pro-Kremlin forces.
"The supply line is cut" for the Ukrainian army, he said.
The official warned that the latest events in Ukraine "have made clear that the security paradigm in Europe has fundamentally changed" in the face of a "very aggressive Russia".
He said the past weeks have seen a "real upsurge in Russia's activity" in the flashpoint region, including the supply of weapons, ammunition, special forces training, intelligence and logistical support.
"All this has been systematically denied, adding confusion," he said.
The reports by NATO come a week before an alliance summit in Cardiff, where possible action against Russia over the crisis in Ukraine will top the agenda.
Ukraine's ambassador to the European Union on Thursday called for "large scale" military assistance from the West as reports emerged that Russian troops had helped open a new front in southeastern Ukraine.
EU leaders will discuss the developments at a summit on Saturday that is primarily tasked with filling top EU jobs.
Immediately preceding the summit, Ukrainian President Petro Poroshenko will meet European Commission head Jose Manuel Barroso in Brussels, as well as EU President Herman Van Rompuy.