More than 3,000 combat troops and hundreds of armoured vehicles are inside Ukraine, a senior official has told Sky News.
"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 said.
He added that around 20,000 Russian troops remained close to the Ukraine border.
Sky News' Defence Correspondent Alistair Bunkall said: "A senior official told me there are now more than 3,000 Russian troops with tanks in eastern Ukraine and in their words, they will soon outnumber the so-called rebels.
"That sums up the task that is facing NATO. Russia has a very formidable military and taking them head-on just isn't an option."
The claim comes as Ukraine's President Petro Poroshenko has said he will order a ceasefire at noon UK time on Friday for Ukraine forces, providing a document is signed in Minsk that sets out a stage-by-stage peace plan.
Senior rebel leaders responded by claiming they would also give orders for a ceasefire if an agreement in Minsk is reached.
Earlier, Sky News filmed what appeared to be well-armed troops wearing recent-issue Russian combat gear and operating alongside pro-Russian rebels in south-eastern Ukraine.
The men were travelling in a small convoy, which included an armoured personnel carrier and two machine gunners mounted on pick-up trucks.
There have also been reports of explosions and columns of smoke in the region.
Speaking from the city of Mariupol, Sky's Katie Stallard said: "We can see smoke rising from the horizon and heard loud explosions that seem to be coming from two distinct directions.
"This is next to a village where we saw Ukrainian troops yesterday digging a second line of trenches and machine-gun positions.
"The soldiers posted on the outskirts are on alert and are refusing to allow us out of the city.
"We're hearing reports that people have seen a column of vehicles that were marked with the white and pink circles of the separatists. They believe it is a column of roughly 50 rebel vehicles heading west towards Mariupol."
Meanwhile, Russia's Foreign Minister Sergei Lavrov accused the US of taking an anti-Russian stance at a time when a ceasefire deal was within reach.
"The surge in anti-Russian rhetoric that we have seen exactly when there is a very active effort to seek a political solution shows that the party of war in Kiev has active external support, in this case from the United States," Lavrov told a news conference.
In contrast, he said, Russia was "doing and will do" everything in its power to secure peace in the region.
Meanwhile, as protests took place outside the venue in South Wales, David Cameron has opened the NATO summit declaring that Russian troops are in Ukraine illegally.
"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."
Source: Sky News