With the established global order on shaky footing, President Donald Trump's week-long trip to Europe will test already strained bonds with some of the United States' closest allies.

Trump has shown little regard for America's traditional bonds with the Old World, publicly upbraiding world leaders at NATO's new headquarters a year ago for not spending enough on defence.

In the run-up to his upcoming trip, the president did little to ease European concerns by delivering fresh broadsides against the inter-governmental military alliance of 29 North American and European countries aimed at countering possible Russian aggression.

A senior official at the German Marshall Fund, a US non-partisan public policy think tank, told The Associated Press that an "unstable, insecure Europe" affects American security and prosperity.

Ian Lesser, who is vice president and executive director at the German Marshall Fund, said there had been many rocky periods in trans-Atlantic relations, but "nothing maybe quite so vigorous" as the current climate.

"The relationship matters," he added.

Experts fear the trip could produce a repeat of the dynamics from Trump's last trip abroad, when he admonished Group of Seven allied nations at a summit in Canada before heading to Singapore, where he showered praise on one of America's longest-standing adversaries, North Korea's Kim Jong Un.

Lesser described Trump's visit to the G7 as "a disaster at many levels".

"It simply was, in retrospect, something, a meeting people probably would have preferred not to have, and this is not the NATO tradition," he said.

Trump is expected to continue to press NATO nations to fulfill their commitments to spend two percent of their gross domestic product on defense by 2024.

The US president has argued that countries not paying their fair share are freeloading off America and has threatened to stop protecting those he feels pay too little.

NATO estimates that 15 members, or just over half, will meet the benchmark by 2024 based on current trends.

Trump sent letters to the leaders of several NATO countries ahead of his visit, warning that it would become "increasingly difficult to justify to American citizens why some countries fail to meet our shared collective security commitments."

The ties between the US and many of its longest-standing allies have frayed since Trump took office and put his "America first" agenda into practice.

He has pulled the US out of the Paris climate agreement as well as the Iran nuclear deal, slapped tariffs on steel and aluminum imports, and threatened additional tariffs on products like automobiles.