"I think in 15 years, it's not inevitable, but it is a very strong likelihood that we will be at war with China," recently retired US Lieutenant General Ben Hodges told defence experts meeting at the Warsaw Security Forum in the Polish capital on Wednesday.

Hodges served as US Army commander in Europe until last year and is now a strategic studies expert at the Washington-based Centre for European Policy Analysis (CEPA).

"The United States does not have the capacity to do everything it has to do in Europe and in the Pacific to deal with the Chinese threat," he said, also describing the US commitment to NATO as "unshakeable".

NATO manoeuvres at the end of October in Norway are intended as a show of force and unity in the face of an increasingly assertive Russia.

Running from October 24-November 7, the Trident Juncture 18 exercise will draw in 45,000-50,000 troops -- the biggest movement of NATO personnel and vehicles since the Cold War.

"It's in the American interest -- and the American leadership knows that -- to have a very strong European pillar. Even if not one European country is spending a euro on its own defence, stability and security in Europe is in the interest of the United States," Hodges later told AFP on the margins of the security forum.

"The president (Donald Trump) knows that, Defence Secretary (James) Mattis knows that, so you're going to see us continue to invest here in Europe, continue to train, to practise rotational forces as well as permanently assign forces for the eventuality that in ten or 15 years we're going to be having to fight in the Pacific," Hodges added.

NATO allies have beefed up their presence on the bloc's eastern flank over the past four years, operating rotating garrisons in eastern Europe and the Baltic states in a response to Russia's 2014 annexation of the Crimean peninsula.

The US army also set up a new European headquarters in Poland in the spring of last year to command some 6,000 of its troops deployed in NATO and Pentagon operations across the alliance's eastern frontier.

The move is one of the largest deployments of US forces in Europe since the Cold War.