The Daily Telegraph By

Russian forces have rehearsed the invasions of Norway, Finland, Sweden and Denmark during a military exercise involving 33,000 troops, according to a study of Baltic security.

The manoeuvres, which took place in March, assumed that a Western-backed uprising against President Vladimir Putin was taking place in Moscow. Under the scenario posited by the exercise, Russia responded by launching a simulated assault on four regional neighbours.

Troops practised attacking Norway with the aim of seizing an area in the north while other forces rehearsed the capture of the Aland islands from Finland. Units also drilled on seizing Gotland island from Sweden and Bornholm island from Denmark.

These Baltic territories lie across vital shipping lanes, making them key military objectives. The capture of these islands would allow Russia to seal the Baltic and isolate Estonia, Latvia and Lithuania.

Edward Lucas, the senior vice-president of the Centre for European Policy Analysis and the author of the report, wrote: "If carried out successfully, control of those territories would make it all but impossible for Nato allies to reinforce the Baltic states."

Of the countries targeted, Denmark and Norway are members of Nato, while Finland and Sweden are officially neutral.

Mr Lucas argues that all four should enhance their military cooperation with other vulnerable states, particularly Estonia, Latvia, Lithuania and Poland. Russia is carrying out a regular series of military exercises near the borders of Nato countries, involving land, sea and air forces.

Nato has responded with drills, including "BALTOPS 2015", an exercise in the Baltic this month involving 49 warships from 14 countries. Significantly, Finland and Sweden chose to join these manoeuvres as Nato "partner" countries.

The Kremlin's military actions betray a preoccupation with achieving dominance of the Baltic - and a willingness to use nuclear weapons. In June last year, Russian jets simulated a nuclear attack on Bornholm, timed to coincide with an annual festival on the Danish island involving the country's entire political leadership and 90,000 guests.

"Had the attack actually taken place, Denmark would have been decapitated," writes Mr Lucas.

In addition, Russian bombers routinely probe the air defences of Nato countries, forcing jet fighters to be scrambled. The presence of Russian intruders, who switch off the "transponder" devices that aircraft use to detect one another, has led to near-misses with civil airliners.

But Russia would probably be unable to mount a real attack on Baltic neighbours for as long as its forces are fighting in eastern Ukraine. American officials believe regular troops are "integrated" with separatist rebels in Ukraine, providing them with weapons, training, logistics and command and control.

Russia and Ukraine have signed the Minsk agreements, providing for a ceasefire. But the bloodshed in Donetsk and Luhansk has escalated in recent weeks, with pro-Russian forces pressing closer towards the port city of Mariupol.

Daniel Baer, the US ambassador to the Organisation for Security and Cooperation in Europe, said observers reported more fighting, raising fears of an escalation. Last weekend, monitors noted 700 explosions in eastern Ukraine. He added: "You can't sustain that without a state-sponsored procurement system, without a logistics system and without resupply. What is happening could not be happening without Russian participation."