By David Brown
Russia has positioned about 130,000 troops – equipped with everything from tanks and artillery to medical facilities and logistical support – close to Ukraine’s border.
They include about 30,000 troops taking part in military exercises in Belarus.
US officials say Russia has the troops in place to invade Ukraine “at any time“.
But Russia denies it is planning an attack.
Troops on the move
Reports suggest that key support units necessary for an invasion have been moving into position alongside combat troops in recent days.
They are thought to include tank repair workshops, mud clearance equipment and field hospitals with blood supplies in some areas.
Some analysts say the presence of manned field hospitals, in particular, could be a key indicator of readiness to attack.
It is unclear whether all the support necessary for an invasion is present in every area.
Units arriving from across Russia have been added to about 35,000 military personnel who are permanently stationed close to Ukraine’s border.
Some newly arrived units have travelled almost 4,000 miles from the Russian Far East.
Much of the heavy armour has been brought in by rail, some passing through Kursk, approximately 80 miles (130km) from the Ukraine border.
Other vehicles have arrived by road, via Karachev in the Bryansk region.
Most estimates have put the number of Russian troops deployed around Ukraine at 100,000 or more.
On Thursday, western officials said the number had reached 130,000, up from their previous estimate of 100,000.
The UK defence secretary Ben Wallace said that Russia had deployed “half its combat troops” near Ukraine’s border or in Belarus.
Ukraine has suggested similar figures, with 112,000 ground troops plus about 18,000 navy and air force personnel in position.
In addition to the regular Russian troops, there are thought to be about 15,000 Russian separatists in Ukraine’s Luhansk and Donetsk regions. Ukraine believes the figure is higher.
In December, a US intelligence assessment suggested the total number of troops could be rapidly increased to 175,000.
Many experts believe that a full-scale invasion, followed by an occupation of most or all of Ukraine, would require many more troops than Russia has currently assembled.
The view from above
The presence of troops can sometimes be detected in satellite images by the colour of tents.
Those occupied are heated, melting the snow on their roofs and appearing darker from above.
Some training exercises with main battle tanks are clearly visible.