NATO has released satellite images it says show Russian combat forces operating in Ukraine in support of separatists.
Earlier Ukraine’s President Poroshenko held an emergency security meeting, saying Russian troops had been deployed.
Moscow has strongly denied the claims, its defence ministry insisting they bear “no relation to reality”.
“We have noticed the launch of this informational ‘canard’ and are obliged to disappoint its overseas authors and their few apologists in Russia,” a ministry official, General-Major Igor Konashenkov, told Interfax news agency.
But according to NATO more than 1,000 Russian troops are in Ukraine.
“These latest images provide concrete examples of Russian activity inside Ukraine but they are only the tip of the iceberg in terms of the overall scope of Russian troop and weapons movements,” said Brigadier General Nico Tak, Director of NATO’s Comprehensive Crisis and Operation Management Centre (CCOMC).
Ukraine’s defence ministry released a video purportedly showing a Russian tank on its territory.
Pro-Russian rebels have regained ground in southern Ukraine and captured a key coastal town.
One separatist leader admitted Russian citizens were fighting amongst them.
Meanwhile fighting has continued in the rebel stronghold of Donetsk, where a government offensive has recently been trying to push out separatists.
The city’s administration said 15 civilians were killed in shelling on Thursday.
More than 2,000 people have died in four months of fighting in Ukraine.