Russia has launched unprecedented land, air and sea drills in annexed Crimea in a coordinated training exercise involving thousands of troops.

The action is billed as significant by the military as it is said to be the first time in the history of the Russian army that three large Airborne units have been “simultaneously alerted”.

The commander in charge of the exercise, Colonel-General Andrei Serdyukov, said the drill was prompted by an "increased terrorist threat” in the region.

It will see more than 2,500 paratroopers and 600 pieces of equipment deployed to the disputed peninsula, which was under the control of the Ukrainian government until Russia illegally annexed it in February 2014.

Russian troops will practice amphibious landings and airdrops and carry out firing drills during “offensive and defensive operations”.

Col Serdyukov said: "For the first time in the Russian army’s history the Airborne Forces’ three large units were simultaneously alerted as part of the drills and partly redeployed to Crimea with weapons and equipment.”

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Western nations continue to condemn the annexation of Crimea as a blatant breach of international law.

Nato, of which Ukraine is not a member, expressed alarm at having not been informed of the exercise, calling it illegal.

Spokesperson Oana Lungescu told The Independent: “Any Russian military exercises in occupied Crimea are illegal under international law as they do not have the consent of the Ukrainian government.

“Since 2014, Russian military activity in the Black Sea region has increased significantly. Russia's wide-ranging military build-up in Crimea poses a challenge to regional stability and international security.

“In response to Russia’s military build-up, Nato has increased its military presence in region.

“This is being done in a defensive and proportionate way and is fully in line with our international obligations.”

Russian military activity in Crimea Show all 11 1 /11 Russian military activity in Crimea Russian military activity in Crimea Ukraine-russian13-ap.jpg Russian, right, and Ukrainian navy sailors are deployed outside a Ukrainian Coast Guard base in Balaklava near Sevastopol, Crimea (AP) AP Russian military activity in Crimea Ukraine-russian9-ap.jpg An unidentified armed man patrols a square in front of the airport in Simferopol, Ukraine (AP) AP Russian military activity in Crimea Ukraine-russian12-ap.jpg A soldier rests atop a Russian armored personnel carriers with a road sign reading "Sevastopol - 32 kilometers, Yalta - 70 kilometers", near the town of Bakhchisarai, Ukraine (AP) AP Russian military activity in Crimea Ukraine-russian11-rtp.jpg Armed Russian navy servicemen surround a Ukrainian border guard base in Balaclava, in the Crimea region (Reuters) Reuters Russian military activity in Crimea Ukraine-russian10-afpgt.jpg Unidentified soldiers block a road to Ukrainian military airport Belbek not far from Sevastopol (AFP/Getty Images) AFP/Getty Images Russian military activity in Crimea Ukraine-russian7-rt.jpg Sea gulls perch onboard a Russian military vessel anchored at a navy base in the Ukrainian Black Sea port of Sevastopol (Reuters) Reuters Russian military activity in Crimea Ukraine-russian6-afpgt.jpg Activists of the Russian Bloc party guard the road to Ukrainian military airport Belbek not far from Sevastopol (AFP/Getty Images) AFP/Getty Images Russian military activity in Crimea Ukraine-russian8-rt.jpg Ukrainian police walk near the Crimean parliament building in Simferopol. Ukraine said on Thursday it would regard any movements by Russian military in Crimea outside the Russian Black Sea fleet's base in Sevastopol as an act of aggression. (Reuters) Reuters Russian military activity in Crimea Ukraine-russian-navy1-rt.jpg Armed Russian navy servicemen surround a Ukrainian border guard base in Balaclava, in Crimea region (Reuters) Reuters Russian military activity in Crimea Ukraine-russian4-ap.jpg An unidentified gunman holds his assault rifle ready while he and others block the road toward the military airport at the Black Sea port of Sevastopol in Crimea (AP) AP Russian military activity in Crimea Ukraine-russian3-ap.jpg A local resident smiles preventing people from going too close to unidentified gunmen blocking the road toward the military airport at the Black Sea port of Sevastopol in Crimea (AP) AP

The drills come amid a build-up of North American and European troops along Russia’s border.

From next month, a US-led battalion of more than 1,100 soldiers – including 150 British personnel - will be deployed in Poland.

US Army Lt. Colonel Steven Gventer described the movement as “a mission, not a cycle of training events”.

Putin joins celebration marking takeover of Crimea

Britain, Canada and Germany are leading the other three battle groups in Estonia, Latvia and Lithuania, which are due to be operational by June.

In total, around 4,000 Nato troops - equipped with tanks, armoured vehicles, air support and hi-tech mission information rooms - will monitor for and defend against any potential Russian incursions.

The alliance is seeking to show ex-Soviet countries that they are protected from the kind of annexation Russia orchestrated in Crimea.