The thorny question of "burden sharing" -- who foots the bill for defending Europe -- loomed large at a meeting of NATO defence ministers in Brussels, but announcements of increased European spending, particularly by Germany, look to have at least eased tensions with Washington.

US President Donald Trump has repeatedly taken aim at European countries, particularly Germany, for not spending enough on defence, accusing them of leaving the United States to shoulder an unfair burden in NATO.

Currently only three European countries are meeting a pledge made at the NATO summit in 2014 to spend two percent of GDP on defence -- Britain, Greece and Estonia.

But alliance chief Jens Stoltenberg announced European NATO members and Canada are on course to increase their defence spending by 3.82 percent this year -- the fourth consecutive annual rise.

This, coupled with the news that Germany is increasing its military budget by three billion euros next year, and will have boosted it by 80 percent by 2024, have gone down well with the Pentagon chief, who hailed "significant progress" by the alliance.

"Last year we... saw the largest cross-NATO increase in military spending in a quarter century," Mattis said after two days of talks with the 28 other NATO defence ministers.

"Many allies are making investments beyond the monetary aspect of the Wales pledge, answering secretary general Stoltenberg's call to provide 'cash and capabilties and contributions."

The two-day meeting at NATO's new headquarters came with the US and Europe at loggerheads over a range of key international issues, from new US tariffs on steel and aluminium to Trump's withdrawal from the Iran nuclear deal and the Paris climate accord.

French Defence Minister Florence Parly told AFP that European countries and Canada had chosen to keep a lid on their unhappiness about these issues in order to ensure successful preparations for the July 11-12 NATO summit.

"We prepared the best we can for the summit, which everyone knows will take place under tension," Parly told AFP.

"We know how strong these tensions are."

Several ministers are understood to have shared their concerns with Mattis over the way the Trump administration has treated its European allies, particularly the fact that "national security" was used as justification for hitting steel and aluminium imports with tariffs.