US Secretary of State Mike Pompeo trekked Wednesday to the Polish town of Orzysz to witness NATO live-fire exercises in an unmistakable sign to Warsaw that the United States has its eye on Russia.

Pompeo walked through drizzling rain with senior Polish officials over frozen soil and patches of snow to a reviewing stand as several hundred troops stood at attention.

Pompeo, a West Point graduate who served as a US Army cavalry officer in Germany when Poland was on the other side of the Iron Curtain, said little had changed except, he joked, that back then he was listening to cassette tapes of Van Halen.

The US defense chief likened Orzysz, around 70 kilometres (45 miles) from Russia's Baltic Sea enclave of Kaliningrad, to divided Germany's infamous Fulda Gap -- where NATO feared that Soviet tanks could penetrate its defenses.

"Today, the gap in which we stand occupies the same priority focus for NATO commanders that the Fulda Gap did back then -- once again because of Russian aggression," Pompeo said in reference to the so-called Suwalki Gap.

He said that the concern was not theoretical, denouncing Russian President Vladimir Putin for the 2014 annexation of the Crimean peninsula and ongoing support for separatists in Ukraine, as well as Moscow's war a decade earlier with Georgia.

"We take seriously those concerns that Russia may one day try to open a front along a line right here," Pompeo said.

- Simulated battle -

Pompeo then advanced with senior Polish officials including Foreign Minister Jacek Czaputowicz to observe exercises by troops from the five countries at the base -- the United States, Poland, Britain, Croatia and Romania.

Amid startling shots from rocket launchers, 20 troops sprinted from a corner for a display of hand-to-hand combat, simulating punching and kicking one another as if in the throes of battle.

Pompeo watched with a smile as Bradley armoured vehicles rolled by and a dozen soldiers jumped out before him and squatted with their rifles, shooting metallic targets in the shape of enemy bodies.

A Polish officer provided a running, dispassionate narrative in precise English, explaining, "The snipers wait patiently for the enemy soldier to present the opportunity for the perfect shot."

- Wooing the US -

Putin has accused the United States of trying to contain Russia and has pledged to boost Moscow's own military, including by deploying nuclear-capable Iskander missiles to Kaliningrad.

Meanwhile, US President Donald Trump has voiced admiration for Putin and mused that NATO -- which considers an attack on any of its 30 members an attack on all -- cheats the United States out of money.

Believing a permanent US presence would be the ultimate deterrent, Poland has offered to contribute to building a US base -- which it has cheekily suggested could be called Fort Trump.

Asked about a troop increase, Pompeo said the United States was "taking a look at it".

NATO promised Russia in 1997 not to station significant forces in the former eastern bloc. As tensions have grown however, the alliance has instead rotated troops through front-line countries.

Poland offered an additional sweetener on Wednesday. US Vice President Mike Pence, also in the country, said Warsaw would buy $414 million worth of US mobile rocket launchers -- good news for the transaction-minded Trump.