U.S. President Donald Trump, left, speaks with Theresa May, U.K. prime minister, as world leaders gather for a family photograph during the North Atlantic Treaty Organization (NATO) summit at the military and political alliance's headquarters in Brussels, Belgium, on Wednesday, July 11, 2018.

U.S. President Donald Trump told NATO leaders on Wednesday they should increase their defence spending to 4 percent of their country's economic output, double the group's current goal of two percent.

NATO allies shrugged off the demand as part and parcel of Trump's brash push for allies to spend more on their own defence at a summit in Brussels, with a quip from the alliance's chief that it should aim to meet its goal before reaching further.

"We should first get to 2 pct," NATO Secretary-General Jens Stoltenberg, adding that eight of the 29 allies were meeting that target, while others had a plan to do - turning a leaf on years of defence budget cuts.

Striking a strident tone at the summit, Trump's aspirational target of 4 percent of gross domestic product (GDP) was above the United States own spending on defence.

The United States, the world's biggest military power, spent some 3.57 percent on defence last year, according to NATO figures.