Russia warns it'll take 'extra measures' if NATO sends men to Ukraine

Russia today vowed to 'take extra measures' if Nato responds after the Kremlin dispatched 4,000 troops to reinforce the disputed Ukrainian border.

'No doubt such a scenario would lead to a further increase in tensions close to Russia's borders. Of course, this would call for additional measures from the Russian side to ensure its security,' Kremlin spokesman Dmitry Peskov told reporters.

He insisted, however, that Russia was 'not threatening' Ukraine, despite an earlier statement which warned a war in Donbass would 'destroy' its neighbour.   

Footage has emerged showing shows dozens of military helicopters close to the frontier, and the movement of tanks and other military vehicles by train.

The United States pledged its 'unwavering' support to Ukraine on Thursday as defense officials said they were aware of thousands of troops newly-deployed to the separatist Donbass territory, which is controlled by pro-Moscow factions.

Kiev has put its forces on high alert with drills close to the rebel controlled Luhansk and near Russian-held Crimea, claiming it is responding to visible aggression by Vladimir Putin's military machine.  

One video shows 2S19 Msta-S tanks - 152.4 mm self-propelled howitzers - by train bound for Crimea, annexed by Putin from Ukraine in 2014.

As the West marks Easter, Ukrainian intelligence sources said there is rising activity from Russian forces in Bryansk, Voronezh, Rostov regions and in Crimea.

Kiev claims Putin has 28 battalion tactical groups along the Russian-Ukrainian border, in Donbass and Crimea.

This is soon expected to be supplemented by up to 25 more battalion groups.

The build-up is said to be Russian war games but it has escalated tensions.

The Kremlin's veteran foreign minister Sergei Lavrov warned against Ukraine and the West provoking a 'hot conflict'.

'Russian President Putin said not long ago, but this statement is still relevant today, that those who would try to start a new war in Donbass will destroy Ukraine,' he said.

'The military understands the harmfulness of any action to unleash a hot conflict. I very much hope that they will not be prompted by politicians, who, in turn, will incite the West, led by the United States.'

Ukraine alleges Russia has 32,700 troops in Crimea, and commands 28,000 separatist servicemen in rebel-held eastern Ukraine.

'Despite significant enemy forces, the Ukrainian army is prepared for all potential scenarios,' said Ruslan Khomchak, the Ukrainian commander-in-chief.

Putin's spokesman Dmitry Peskov defended the troop movement towards border areas.

The Russian Federation moves troops within its own territory at its own discretion,' he said.

'This shouldn't concern anyone, as such actions do not endanger anyone.'

He claimed: 'As you know, intense activity of the armies of NATO member states and other organisations, as well as particular states etc, is being observed on the perimeter of Russian borders.

'We need to stay on guard.'

A NATO official said that the Western allies have 'concerns' over Russian 'large scale military activities around Ukraine'.

An American expert on Russia, Michael Kofman said that while there was 'no strong evidence that an attack is imminent' it was clear 'something is up outside of regular exercises or normal troop rotation'.

He warned: 'Russian intentions are unclear.'

Ukraine, Western countries and NATO accuse Russia of sending troops and heavy weapons to prop up its proxies in Donbass who seized a swathe of eastern Ukraine in 2014.

Russia says it only provides political and humanitarian support to separatist fighters in what it casts as an internal conflict.

Ukrainian military intelligence accused Russia of engineering provocations to create a pretext to send additional Russian military units to Donbass.

'In addition, an attempt to advance Russian occupation forces deep into Ukraine is not ruled out,' it said.