FILE - In this May 7, 2018 file photo, NATO Secretary General Jens Stoltenberg, center, walks in the Agora Hall as he arrives for his first day of work at the new NATO headquarters in Brussels. Stoltenberg is playing down differences among member countries as the military alliance marks its 70th anniversary in Washington on April 4, 2019. (AP Photo/Virginia Mayo, File)

BRUSSELS - The United States and its allies are stepping up co-operation in response to Russian aggression, NATO Secretary-General Jens Stoltenberg said Monday while playing down differences among members as the military alliance marks its 70th anniversary.

Foreign ministers from NATO countries are meeting in Washington this week for the occasion, determined to show a united front in the midst of a long military stalemate in Afghanistan and tensions with Russia returned to Cold War era levels.

But as NATO deploys thousands of troops and equipment to deter Russia and seeks solutions to fast-evolving new threats such as cyberattacks and hybrid warfare, its biggest challenge arguably lies within. Damaging infighting over defence spending and authoritarian tendencies exhibited by some allies undermine NATO's values, according to experts.

"The strength of NATO is that despite these differences, we have always been able to unite around our core tasks. That is, to protect and defend each other," NATO chief Stoltenberg said in Brussels before the trip.

Stoltenberg has talks with U.S. President Donald Trump planned for Tuesday. He is scheduled to address Congress on Wednesday.

A big source of the internal strain is Trump's recurrent demand that countries devote an amount equal to at least 2 per cent of GDP to defence spending — though that metric takes no account of how well the money is spent — as well as the U.S. president's reluctance to criticize strongmen like Russian President Vladimir Putin.

"NATO's single greatest challenge is the absence of strong, principled American presidential leadership for the first time in its history," two former U.S. envoys to NATO, Nicholas Burns and Douglas Lute, wrote in a report for the alliance anniversary.

Trump, they said, is seen by allies as NATO's "most urgent, and often most difficult, problem."

Secretary of State Mike Pompeo testified last week before the House Foreign Affairs Committee in Washington. He was asked if he was familiar with the report by Burns and Lute.

Pompeo said he hadn't read the report but he had known Lute since he was a young military office and had "great respect for him." But on the subject of Trump and NATO, Lute was "just simply wrong," Pompeo said.

If the conclusion he drew that President Trump is the biggest impediment to NATO, he's just simply wrong," he added. "We have worked diligently to make NATO stronger. I am convinced that we have done so."

Trump made a memorable impression on leaders from Canada and European nations during his first NATO summit in May 2017. During a speech outside NATO's new Brussels headquarters, he publicly humiliated them. Trump also cast doubt on whether they could count on Washington to fulfil NATO's collective defence clause.

The speech was delivered by a memorial made from a twisted piece of the World Trade Center towers felled by al-Qaida's airliner attacks on Sept. 11, 2011. Since the founding Washington Treaty was signed on April 4, 1949, NATO has only once activated the clause stating that an attack on one member is an attack on them all, after the 9-11 attacks.

Trump also delayed a summit last year with fresh demands on burden sharing. That time, at least, his dressing down about the U.S. spending more on defence than the other NATO members combined happened behind closed doors.

Trump's routine tirades have fueled suspicion his aim mostly is to drum up business for the U.S. defence industry.

But the attitude of Trump — who walked away from the Iran nuclear deal and the Paris climate change agreement his allies in Europe value while making tariff threats against them — also is similar to authoritarian or populist streaks showing up in NATO members Turkey, Hungary, Italy and Poland.

"The political and trans-Atlantic unity that underpins NATO has been weakened. Only bad guys benefit from trans-Atlantic division and a U.S. retreat from its global leadership role," Anders Fogh Rasmussen, Stoltenberg's predecessor as NATO's top civilian official, said in an email exchange with The Associated Press.

"We see the consequences of U.S. retreat, with autocrats and dictators filling the vacuum," he added,

Burns and Lute say this retreat from democracy, individual freedoms and the rule of law is "a potentially cancerous threat."

Still, NATO has survived formidable challenges over the decades, including the Cuban missile crisis and the missile race in Europe. It's also remained intact after internal divisions over the Suez Canal, the Iraq war, and France's departure from the alliance's command structure. Officials say they are confident NATO will endure now, too.

The White House said last month in a statement about Stoltenberg's talks with Trump on Tuesday that the two would "discuss the unprecedented success of NATO, including the recent increased commitments on burden-sharing among European allies and ways to address the current, evolving challenges facing the alliance."

Trump is not scheduled to appear at the upcoming talks of NATO foreign ministers, but he is expected to attend a leaders' summit in London in mid-December.

"I hope we will not see a repeat of President Trump's antics in Brussels last year. It's time for the world's democracies to show their unity," Rasmussen said.

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Deb Riechmann contributed from Washington.

News from © The Associated Press, 2019