Gregory Korte

USA TODAY

WASHINGTON — The Secretary General of NATO and the president of its most powerful member nation both proclaimed the importance of the treaty organization on its 67th birthday on Monday, tacitly defending the alliance from Donald Trump's criticism that it's outlived its usefulness.

"NATO continues to be the linchpin, the cornerstone of our collective defense and U.S. security policy," President Obama said after a White House meeting with NATO Secretary General Jens Stoltenberg Monday.

"NATO is as important as ever, because NATO has been able to adapt to a more dangerous world," Stoltenberg said. "We stand together in the fight against terrorism. Terrorism affects us all, from Brussels to San Bernardino,"

Trump, the New York real estate mogul who's leading in the delegate count for the Republican presidential nomination, said last week that the alliance has become obsolete and should be "rejiggered." He later said that nations that aren't paying their fair share — defined under the treaty as 2% of their economy for defense — should be kicked out of the alliance. "If it breaks up NATO, it breaks up NATO," he said.

Obama: Trump doesn't understand foreign policy or the world

Afterward, Stoltenberg would not comment on Trump's comments directly. "I will not comment on the election campaign in any NATO-allied country. It's up to the American people to decide," he told reporters outside the White House.

But he did say the alliance has made slow progress on making sure member nations pay their fair share. "The picture is mixed, but the picture is better than it was a couple of years ago," he said. "A lot of allies still have a lot to do when it comes to defense spending."

NATO was founded April 4, 1949, with 12 charter members: the United States, Canada, and 10 western European nations. It's since expanded to 28 member countries — including 12 countries that were formerly behind the Soviet Union's so-called Iron Curtain.

Trump said last week that NATO “doesn't cover terrorism," — only the Soviet Union, "which is no longer in existence.”

But Obama and Stoltenberg said that while their agenda included the Russian incursion into Ukraine, they also discussed Afghanistan, Iraq, Syria and Libya — what Obama called "the southern flank" of NATO.

Among the provisions of the treaty is Article 5, which holds that an attack on one member country is an attack on all of them. That provision has been triggered only once, after the 2001 terrorist attacks on New York and Washington, and led to a NATO force in Afghanistan that remains today.

White House Press Secretary Josh Earnest downplayed the importance of Trump's position in Obama's talks with the NATO leader.

"This is something that has been on the books long before Mr. Trump's ill- advised comments about the importance of the U.S.-NATO relationship," he said. "I would be very surprised if there was any extensive conversation that involved Mr. Trump in the meeting."