A senior Nato official has claimed that 3,000 Russian combat troops and hundreds of armoured vehicles are operating inside Ukraine.
"We are still seeing several thousand Russian combat troops on the ground inside Ukraine, equipped with hundreds of tanks and armoured vehicles, so (there is) no substantial change in the disposition of Russian forces inside Ukraine," the official told Sky News.
The unnamed official added that approximately 20,000 Russian troops remain stationed near the Ukraine border.
The claim by Nato comes after Ukraine's President Petro Poroshenko announced that he is to order a Ukrainian ceasefire on Friday at 11.00am (GMT) if a step-by-step peace plan is signed at talks in Minsk.
Senior separatist leaders have also signalled that they will order troops to put down their weapons if the agreement is signed in the Belarussian capital.
Before the talks, Russian Foreign Minister Sergei Lavrov warned Nato against offering Ukraine membership of the military alliance as its leaders gathered for the summit in Wales.
"It is precisely at such a moment when a chance has emerged to start solving specific problems between Kiev and the militias that some sections of the Kiev authorities make demands for Ukraine to drop its non-aligned status and start joining Nato," Lavrov said at a meeting with the Organization for Security and Cooperation in Europe (OSCE).
"It's a blatant attempt to derail all efforts aimed at initiating a dialogue on ensuring national reconciliation."
British Prime Minister David Cameron opened the summit, reaffirming the claim that Russian forces are operating illegally in Ukraine.
"We meet at a crucial time in the history of our alliance," he said. "The world faces many dangerous and evolving threats. Nato is as vital to our future as it has been in our past."