European NATO allies and Canada will increase defence spending this year by 4.3 per cent, alliance chief Jens Stoltenberg said on Wednesday, amid pressure from President Donald Trump to spend more.

"In 2017 we foresee an even greater annual real increase of 4.3 per cent. That is three consecutive years of accelerating defence spending," Stoltenberg told reporters on the eve of a Defence Ministers' meeting in Brussels.

"So we are really shifting gears, the trend is up and we intend to keep it up," he added.

Trump has repeatedly berated the allies for not doing more to share the defence burden and bluntly told them again at a leaders' summit in Brussels last month that they could not count on Washington coming to their defence if they did not do their bit.

Trump's comments caused consternation among many, notably Germany, but Stoltenberg said the President's demands were understandable given the challenges the US-led alliance now faces.

"I welcome the strong focus of Trump on spending and defence burden sharing," he said.

"At the same time, I also underline that allies should invest more in defence not to please the United States but because it is in their own interest and they have made the commitment."

Stoltenberg, a former Norwegian Prime Minister, recalled that the 28 allies had pledged at a 2014 summit in Wales to increase defence spending to the equivalent of two per cent of annual economic output within a decade.

That move, pushed by then president Barack Obama in response to the Ukraine crisis and a more aggressive Russia, had halted and reversed years of defence cuts, Stoltenberg said.

So far only five allies have met that benchmark - the US, Greece, Britain, Estonia and Poland. But Stoltenberg said Romania was set to join them this year, and Latvia and Lithuania in 2018.

In 2015, the allies turned the corner with an increase of 1.8 per cent overall, pushed that to 3.3 per cent in 2016 and now looked to go further again this year, he said.

In all, the three years represented an overall increase of $46 billion dollars, boosting NATO's ability to face the Russian challenge in Europe and new threats such as Islamic State-inspired jihadi terrorism across the Middle East and North Africa.

The United States accounts for about 70 per cent of combined NATO defence spending and Washington has pushed the allies for years to do more to ease the burden.

Trump, however, has pressed hardest of all, putting the allies on the back foot by dubbing NATO "obsolete" and questioning the wisdom of the US security commitment if they failed to live up to their side of the bargain.