Rotterdam harbour on Wednesday saw the offloading of some 60 US Army Black Hawk and Chinook troop transport helicopters, belonging to the 4th Combat Aviation Brigade based out of Fort Carson in Colorado.

The brigade is starting a nine-month rotation to Europe as part of "Operation Atlantic Resolve" which since April 2014 has seen additional US troop and NATO deployments to the continent, aimed at reassuring eastern allies and after Russian actions in Ukraine.

More than 6,000 US troops are currently deployed in NATO members Germany, Estonia, Latvia, Lithuania, Poland, Slovakia, Hungary, Romania and Bulgaria.

"The message is to 27 countries within NATO here in Europe that we're here, we can be relied on, we're friends within that alliance," Colonel Geoff De Tingo told AFP as the helicopters readied for deployment.

"We are practising coming to Europe, faster and faster. We need to come be able to come to the aid of our Dutch allies and NATO faster... in response to anything you would need help with," said De Tingo, chief of staff of the 21st Theatre Sustainment Command, responsible for the move.

US maintenance staff Thursday put together the helicopters at a heavily-guarded shed in the massive Rotterdam port, from where they flew to a Dutch military base near Eindhoven and then to Germany.

The helicopters will eventually be deployed to eastern NATO countries, De Tingo said, adding that a consignment of Apache attack helicopters was also due within the next few weeks.

Last month some 3,000 soldiers from the 1st Armored Brigade Combat Team, 1st Cavalry Division -- the ground contingent of Atlantic Resolve -- arrived in Europe, US military reports said.

The deployments to the region were ordered during the previous administration of president Barack Obama.

The Pentagon's "Atlantic Resolve" operation is aimed at countering security concerns triggered on NATO's eastern flank by Russia's 2014 annexation of Crimea from Ukraine.

But rhetoric by US President Donald Trump, who called NATO "obsolete" and has demanded that allies pay their share in the alliance, deeply rattled its easternmost members bordering Russia.

Trump has since reversed much of his criticism and made good on US troop commitments for the unprecedented NATO deployments across its eastern flank.

The military rotation -- the third since the start of operations -- also comes ahead of a planned NATO summit in Brussels next month which Trump is expected to attend.