Defence Secretary Michael Fallon, speaking aboard HMS Ocean, also said the UK will "stand tall, shoulder-to-shoulder" with other alliance members to defend eastern Europe's borders.

The Royal Navy Flagship is taking part in an amphibious landing exercise on a beach in northern Poland, less than 100 miles from the Russian state of Kaliningrad.

Some 17 NATO militaries including 5,600 troops and 49 vessels were involved.

It is intended to demonstrate solidarity with Eastern Europe in the face of a Russian threat.

But the exercises have been criticised by Moscow, which accused NATO of reviving the ghost of the Cold War by coming so close to its borders.

East-West tensions remain high over the continuing crisis in Ukraine, where Russia-backed separatist rebels seized land in the east after Moscow annexed Crimea from Kiev in early 2014.

And these have increased over Russia's decision to beef up its nuclear arsenal in response to plans by Washington to base heavy military hardware in eastern Europe.

Kremlin spokesman Dmitry Peskov said: "It's not Russia that's approaching someone's borders. It's NATO's military infrastructure that is approaching the borders of Russia.

"All this...forces Russia to take measures to safeguard its own interests, its own security."

Meanwhile, NATO Secretary General Jens Stoltenberg accused Russian President Vladimir Putin of "sabre rattling" after he said Moscow would increase its stockpile of nuclear missiles.