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A retired US four-star general has raised fears Russian President Vladimir Putin is plotting a military invasion of Baltic countries to challenge president-elect Donald Trump and Nato countries.

In a show of force Russia has moved thousands of troops, military equipment and nuclear-capable ballistic missiles to positions in the enclave of Kaliningrad, which is sandwiched between Nato members Lithuania and Poland.

Veteran US military commander Jack Keane said the deployment of troops and equipment to the region suggests Putin may be planning to occupy Baltic states in a move similar to Russia's annexation of the Crimean Peninsula in Ukraine.

Putin has been accused of stoking tensions in Europe and there are fears the escalating situation could spark a new war.

(Image: Getty) (Image: Google)

Mr Keane, who revealed he turned down an offer to become Trump's Secretary of Defense, told The Times: "The Russians are [now] putting considerable pressure on the Baltic states, with the very real possibility of challenging Nato with a military hybrid occupation similar to eastern Ukraine."

He added: "There is a massive military build-up in Kaliningrad, including troops, anti-shipping and anti-aircraft missiles."

Mr Keane suggested Putin is attempting to strip the US and its Nato allies of the "air and maritime advantage" they hold in that region of Eastern Europe.

Nato troops are greatly outnumbered in the region, where it has been building a military defence system.

(Image: Getty) (Image: Getty Images)

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The organisation believes there are around 225,000 Russian troops in Kaliningrad. Russia has staged a number of military drills in the enclave in recent months.

The increasingly unstable security situation is poised to become one of Trump's first major challenges when he takes over the Oval Office in January.

Trump spoke positively of Putin during his successful presidential campaign and the two leaders have discussed issues by phone since the election.

They spoke of bilateral cooperation and common threats and challenges facing both countries, according to statements from their offices.

During his election campaign Trump said he would be open to an alliance with Russia in order to wipe out ISIS in the Middle East.