Poland and its NATO partners on Monday kicked off their largest-ever joint military exercises aimed at shoring up security on the alliance's eastern flank amid the West's worst standoff with Russia since the end of the Cold War.

The two-week long Anaconda manoeuvres are aimed at "checking the alliance's ability to defend its eastern flank," Polish Defence Minister Antoni Macierewicz said at formal opening ceremonies in Warsaw before troops hit the ground on Tuesday.

More than 31,000 soldiers from 24 NATO and former-Soviet "Partnership for Peace" states including Ukraine are taking part in the manoeuvres, held biannually across Poland since 2006.

Some 14,000 US troops will join 12,000 Polish soldiers and around 1,000 from Britain for the exercises involving some 3,000 vehicles, 105 planes and 12 naval vessels.

US Army Chief of Staff Mark Milley said the American presence "demonstrates that we are shoulder to shoulder with the Polish people" and that the exercises would "improve our collective readiness."

They come a month ahead of a "landmark" NATO summit in Warsaw set to seal its largest revamp since the Cold War by deploying more troop rotations in eastern European members spooked by Russia's 2014 annexation of Crimea from Ukraine.

Russia is fiercely opposed to the move, billed by NATO as its "deter and dialogue" strategy.

While NATO cut all practical cooperation with Moscow following Russia's Ukraine intervention, the US-led alliance has said it will hold formal talks with the Russians before the July 8-9 summit.

But just last month Moscow and Washington accused each other of mounting an aggressive military presence in northern Europe as the United States broke ground on a missile shield in NATO allies Poland and Romania.

Russia has vowed to "end threats" posed by the system, despite US assurances that it is intended to ward of potential attacks by so-called "rogue states" in the Middle East.

The Kremlin said it would set up three new divisions in the west and south of the country by the end of the year to counter NATO forces close to its border.

- Hybrid warfare -

Macierewicz said Monday that Polish paramilitary forces will take part in the Anaconda exercises for the first time, part of Warsaw's strategy to counter "hybrid warfare".

That tactic is based on deception rather than a formal declaration of war, NATO strategists say and suggest Russia used it to annex Crimea by covertly deploying unidentified troops.

They also say the same tactic was used to engineer the pro-Moscow revolt in eastern Ukraine that followed.

Macierewicz said last week that Poland will soon enroll the first volunteers in a 35,000-member paramilitary force aimed at parrying a perceived hybrid threat from Russia.

Warsaw will use these new "territorial defence forces" to expand its armed forces next year to 150,000 men from the current 100,000.

Russia has long objected to NATO's expansion in its Soviet-era back yard and in 1997 NATO formally agreed not to install permanent bases in former Warsaw Pact states.

In line with the agreement, the Pentagon said in March it would deploy an additional armoured brigade of about 4,200 troops in eastern Europe from early 2017 on a rotational basis -- not a permanent base.

NATO has been careful to reassure Moscow ahead of the July summit, with its Secretary General Jens Stoltenberg insisting "the Cold War is history and we want it to stay that way."