NATO released Thursday a score of satellite pictures of up to 40,000 Russian troops massed along the Ukraine border that it said are ready for action and represent a "real threat" to Kiev.

The Western military alliance estimates that "between 35,000 and 40,000" Russians, armed with tanks and military vehicles, have been deployed in over 100, mainly temporary, bases to the east of Ukraine.

Brigadier Gary Deakin, director of the Comprehensive Crisis Operations and Management Center (CCOMC) at NATO's SHAPE military headquarters, estimated the troops could be deployed in 12 hours at Moscow's command.

"This force is very capable, ready to move quickly if ordered," he said.

"It's a concern because it represents a real threat for Ukraine," he said as NATO unveiled 19 pictures, most in black-and-white, of several Russian military sites that were shot at the end of March and early April

One shows 21 helicopters on a landing strip "that didn't exist before" troops dug in during early March in the middle of a field near the city of Belgorod, some 40 kilometres from the border.

Around 40 tanks and 100 infantry vehicles are lined up nearby "though there is no trace of a permanent base," the NATO official said.

The fact they were not camouflaged "means that you are ready to go or that you want to be seen," said Deakin.

Other photos show troop concentrations at other parts of the border alongside hundreds of troop transport vehicles, equipment and arms.

Some of the photos highlight the presence of temporary air bases near the Azov sea, between Ukraine and Russia, and close to Crimea. One shows Sukov 27/30 and 24, along with Mig-31 military aircraft ready to take off at an airport, which had been left in disuse until recent weeks.

"They are able to reach Ukraine in a few minutes," said Deakin.

Commenting on whether they were part of a military exercise, he said: "We don't see a lot of exercises going on."