In pictures: Russian military build-up near Ukraine Published duration 11 April 2014

Nato's decision to release over 20 satellite images and associated maps of the Russian military build-up on Ukraine's eastern frontier is a signal of the concern among the alliance's commanders that a Russian military option against Ukraine is very much on the table, the BBC's diplomatic correspondent Jonathan Marcus reports.

The imagery issued by Nato's Supreme Headquarters Allied Powers Europe (Shape) reportedly dates from late March and early April. It encompasses five locations in an arc around Ukraine's frontier. Imagery from a sixth site was provided by Airbus Defence and Space.

The images show:

sophisticated warplanes, and helicopters

an airborne early warning aircraft (similar to the Nato Airborne Early Warning and Control Aircraft, or Awacs)

a number of military deployments on the ground

1. Belgorod

image copyright Digitalglobe via shape image caption This satellite image reportedly from 26 March appears to show Russian Mil Mi-8 Hips and Mil Mi-24 Hinds near Belgorod, Russia

image copyright Digitalglobe via shape image caption Tanks and IFV elements of Motorised Rifle Regiment are shown in this satellite image reportedly from 26 March near Belgorod

2. Buturlinovka Air Base

image copyright Digitalglobe via Shape image caption Satellite imagery of the formerly vacant Buturlinovka Air Base in Russia appears to show a Russian military build-up at three locations

image copyright Digitalglobe via shape image caption This satellite image reportedly from 2 April purports to show Russian Su-27/30 Flankers and Su-24 Fencers at the air base

3. Kuzminka

image copyright AP/DigitalGlobe via SHAPE image caption Russian military tanks and infantry fighting vehicles appear to be shown in this satellite image reportedly from 27 March of a military base near Kuzminka, southern Russia

image copyright Digitalglobe via shape image caption This satellite image from 27 March appears to show tanks and supply elements of a brigade near Kuzminka, close to Ukraine's eastern border

4. Novocherkassk

image copyright Digitalglobe via shape image caption This satellite image reportedly from 27 March appears to show the marshalling of elements of a Motorised Rifle Regiment (MRR) near Novocherkassk, Russia

image copyright AP/DigitalGlobe via SHAPE image caption This image reportedly from 27 March appears to show a Russian artillery battalion at a military base near Novocherkassk, east of the Sea of Azov in southern Russia

5. Yeysk

Among the most interesting of these is what is thought to be an airborne or Spetznaz (Special Forces) brigade at the Yeysk air base on the Sea of Azov some 20km (12 miles) from Ukrainian air space.

image copyright AP/DigitalGlobe via SHAPE image caption This satellite image reportedly from 22 March purports to show a Russian military airborne Special Forces brigade at Yeysk in southern Russia, near its border with Ukraine

image copyright Digitalglobe via shape image caption This image reportedly taken on 22 March appears to show a Beriev A-50 Mainstay airborne early warning aircraft at Yeysk

At a number of other locations the massed vehicles of mechanised infantry or motor rifle brigades can be seen - not just tanks and infantry combat vehicles but artillery and the large number of supply trucks needed to keep a mechanised force on the move.

Before and after

An unnamed Russian military official responded to the release of the images by saying that the satellite photos were actually taken in 2013.

Nato, however, rejects the Russian allegations out of hand.

It says: "The dates of the images released by Shape were collected by the Digital Globe satellite 'Constellation' between late March and early April 2014. The images are unclassified and are commercially available in Digital Globe's public archive. Shape did not alter or edit the images in any way prior to release."

On Thursday Nato released additional images obtained from Digital Globe which show the same locations in 2013 and early 2014 when, Nato says, there was "no evidence of any military activity at all".

image copyright DIGITAL GLOBE VIA SHAPE image caption Nato says this satellite image taken on 17 March shows Russia's military presence near Kuzminka

image copyright DIGITAL GLOBE VIA SHAPE image caption This satellite image of the same site, taken on 25 October, appears to show an emptier site near Kuzminka

A senior Nato spokesman says that when reviewed alongside images released by Shape earlier on Thursday, "it is clear that the military build-up of forces occurred in early March 2014".

He says that Nato "stands firm in its assessment that Russian forces in the vicinity of the border with Ukraine number in the range of 35,000 to 40,000 troops and are equipped with infantry fighting vehicles, tanks, combat aircraft, logistics, and artillery".

6. Primorsko-Akhtarsk

image copyright CNES 2014 Distribution Astrium Services

This 14 March 2014 photo (provided by Airbus Defence and Space) of the Primorsko-Akhtarsk airbase in south-western Russia shows a build-up in the number of tactical aircraft

There are primarily Sukhoi Su-25 Frogfoot strike aircraft (lower left) and Sukhoi Su-27 (on the right)

While possibly deployed to initially be available over Crimea, they are also within range of a lot of eastern Ukraine and represent a threat to any Ukrainian armoured units