Brussels: US Defence Secretary James Mattis on Wednesday bluntly warned Nato allies to meet their spending pledges or else the Donald Trump administration would “moderate its commitment” to the transatlantic alliance.

Mattis’s ultimatum to his counterparts in Brussels follows years of calls by Washington for Nato members to spend two per cent of their GDP on defence, a goal few meet despite agreeing on it at a summit in 2014.

But Mattis’s strongly worded call for cash carries extra weight as it comes after Trump has said US help for Nato allies, already worried by the threat from Russia, might be contingent on how much they have paid.

“Americans cannot care more for your children’s future security than you do,” retired marine general Mattis said in prepared remarks to defence ministers at Nato headquarters in Brussels.

“If your nations do not want to see America moderate its commitment to this alliance, each of your capitals need to show support for our common defence.”

Mattis called for “milestone dates” this year that would track Nato member contributions.

The directness of Mattis’ message took many observers by surprise — he had said his focus during his first Nato summit would be to “listen, learn, help and lead” after Nato allies expressed concerns over Trump’s commitment.

Currently, only the United States, Britain, Estonia, Greece and Poland have hit or surpassed the 2 per cent figure.

Mattis’s visit to Nato, his first trip to Europe since being sworn in last month, comes amid a growing scandal at the White House following the resignation of Trump’s national security adviser Michael Flynn over allegations he had discussed US sanctions with Russia’s ambassador before taking office.

Nato partners are worried about possible ties between other officials on Trump’s campaign team and Moscow, as well as the president’s previous comments that the alliance was “obsolete”.

Despite lambasting some Nato partners’ spending, Mattis hailed Nato as the “fundamental bedrock” of transatlantic security as he sought to reassure allies Wednesday about Trump’s commitment to the alliance.

Since his inauguration, Trump has taken a more orthodox stance on Nato and reaffirmed long-standing US commitment to the alliance.

“As President Trump has stated, he has strong support for Nato,” said Mattis, who has himself previously served with Nato.

Nato chief Jens Stoltenberg said the United States was right to demand that allies pay their way, saying it was his “top priority” in the job.

Stoltenberg said the alliance in 2015 had stopped military budget cuts and last year actually increased spending by 3.8 per cent, or $10 billion (Dh3.67 billion), but still needs to do more.

Stoltenberg also insisted the Flynn scandal was not a further cause for concern for the alliance, which has underpinned transatlantic security since the aftermath of the Second World War.

“I am absolutely certain that the message from this meeting will be a message of transatlantic unity,” Stoltenberg said when asked about Flynn’s resignation.

Mattis said on the flight to Brussels that Flynn’s departure would have “no impact” on the US message to Nato.

He also praised the alliance for its enduring help for the United States in Afghanistan, calling it the “the most successful alliance in military history.”

The Pentagon chief was expected to ask partners for greater troop contributions in the war-torn country, which despite more than 15 years and billions of dollars remains in the throes of a persistent security crisis.

Also hanging over the meeting was a New York Times report that Moscow had deployed a new cruise missile, raising fears it would violate the Intermediate Range Nuclear Forces Treaty (INF).

Like the US State Department, the Nato chief said he would not comment on intelligence matters but warned that “any non-compliance of Russia with the INF treaty would be of serious concern for the alliance.”

Mattis said the United States and Nato would “seek to engage Russia,” but “at the same time defend ourselves if Russia chooses to act contrary to international law.”

Nato leaders are expected to meet Trump for the first time at a summit in Brussels on May 25.