U.S. President Donald Trump's defence secretary warned NATO allies on Wednesday that they must honour military spending pledges to ensure the United States does not "moderate" support for the alliance.
Jim Mattis, on his debut trip to Brussels as Pentagon chief, also accused some NATO members of ignoring threats, including from Russia.
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"America cannot care more for your children's future security than you do," Mattis said in a closed-door session with NATO defence ministers, according to prepared remarks provided to reporters.
The comments represented some of the strongest criticism in memory of allies who have failed to reach defence spending goals.
Europe's low expenditure has long been a sore point for the United States, which puts up 70 percent of alliance funds. But Trump has made change a priority, saying allies have "been very unfair to us" for not spending more.
Trump was sharply critical of NATO during his election campaign, making European allies nervous by calling the alliance obsolete and praising Russian President Vladimir Putin.
NATO allies seek reassurance
Nervous European allies put a brave face on the troubled debut of President Donald Trump's administration, expressing confidence in U.S. engagement with NATO as they prepared to hear from Trump's defense chief at talks in Brussels on Wednesday.
NATO's chief says the military alliance would be concerned if reports that Russia has violated a Cold War-era treaty by deploying a cruise missile prove true.
U.S. intelligence agencies have assessed that the missile became operational late last year, possibly violating the 1987 Intermediate-Range Nuclear Forces Treaty on the development and testing of cruise missiles.
NATO Secretary General Jens Stoltenberg said Wednesday that "compliance with arms control agreements is of great importance and especially when it comes to treaties covering nuclear weapons."
He said that "any non-compliance of Russia with the INF Treaty would be a serious concern for the alliance."
His remarks in Brussels came before chairing the first meeting of NATO defense ministers with U.S. Defense Secretary Jim Mattis.
The meeting at NATO headquarters is seen as an opportunity for Defense Secretary Jim Mattis to reaffirm U.S. commitment to European security after an election campaign in which Trump appeared to question the value of the alliance.
His administration was jolted again this week by the resignation of national security adviser Michael Flynn, who was seen in Moscow as a leading advocate of warmer ties with Russia.
But NATO Secretary General Jens Stoltenberg dismissed concerns about the turmoil or U.S. backing for NATO.
"I'm absolutely certain that the message of this meeting will be a message of transatlantic unity, of the importance of that we stand together and protect each other, and a very strong commitment of the United States to NATO," he told reporters.
Britain's defence secretary echoed that mantra.
"There is no uncertainty about America's commitment to NATO," Michael Fallon told reporters when asked about the Flynn's resignation. Denmark's Claus Hjort Frederiksen said everything he had heard was "calming and reassuring."
Still, European allies are expected to privately seek details from Mattis about Trump's attitude toward the North Atlantic Treaty Organisation, which has guaranteed Europe's security for almost 70 years.
Trump has deeply unsettled allies with his contradictory remarks on NATO - calling it "obsolete" but also voicing support - and with his praise for Russian President Vladimir Putin.
Moscow's 2014 annexation of Crimea, Ukraine's Black Sea peninsula, and its direct support for rebels in eastern Ukraine have prompted NATO to begin sending alliance troops to Russia's borders to deter the Kremlin from any other moves. The White House said on Tuesday that Trump expected Russia to hand back Crimea to Ukraine.
One senior European NATO diplomat said allies are under no illusion that Trump might not still undo careful diplomatic work with a single message on Twitter. But they would still look for his support as they try to manage tensions with Russia.
"We need to bring the family together," the diplomat said. "Things have been made to feel fragile."
In his debut trip to Europe as Pentagon chief, Mattis is set to echo longstanding U.S. calls that European allies invest more on defense, something his predecessors under Republican and Democratic administrations have done for years.
But experts say that message will have to be calibrated, because while Trump has eased off his criticism of NATO since taking office, he has maintained criticism of some allies for failing to make "full and proper financial contributions."
NATO Europe argues it is increasing defence spending, upping outlays by $10 billion last year.
NATO defence ministers from Europe, Canada and Turkey will also want to hear from Mattis, who will be first to address the 27 other defence ministers on Wednesday, on how the alliance can meet Trump's demands to do more to counter Islamic militants.
NATO says it is already active, training troops from Afghanistan to Iraq, and cannot do the work of police or social workers seeking out returning foreign fighters from Syria.
"We need to be clear that the response to international terrorism cannot be led by NATO, but it can a part of that," a second senior European NATO diplomat said.