NATO Secretary General Jens Stoltenberg, in a historic address to Congress, declared Wednesday that President Trump’s push for NATO allies to increase their defense spending has “had an impact” and made the alliance stronger.

“Allies must spend more on defense. This has been the clear message from President Trump,” he said. “And this message is having a real impact.”

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NATO does not have a defense budget, but members commit to spending a minimum of 2 percent of their Gross Domestic Products on defense. Trump, however, has repeatedly called out members that do not meet that commitment, even as this campaign has caused friction with longtime U.S. allies. The United States spends 3.5 percent of its GDP on defense, and Trump has long called for fellow NATO members to put up their share.

As Trump himself has taken credit for nations agreeing to increase their spending pledges, Stoltenberg said that NATO is stronger as a result of the U.S. pressure.

“After years of reducing defense budgets, all allies have stopped the cuts and all allies have increased their defense spending. Before they were cutting billions, now they are adding billions,” he said.

Stoltenberg told lawmakers that European allies and Canada have spent an additional $41 billion in the last two years and that by the end of 2020, that figure will rise to $100 billion.

“That money will allow us to invest in new capabilities our armed forces need, including advanced fighter aircraft, attack helicopters, missile defense and surveillance drones,” he said. “This is good for Europe and it is good for America.”

In a wide-ranging speech in which he talked about NATO’s importance in winning the Cold War and fighting Russian aggression and terrorism, he also praised the U.S. and NATO’s effort to defeat ISIS in the Middle East -- saying the coalition made “remarkable progress.”

“Thanks to American leadership and our collective efforts, we have stopped this brutality and millions of people have been liberated,” he said.

The speech coincides with events marking the 70th anniversary of the alliance's founding charter. Stoltenberg, a two-time prime minister of Norway and the first NATO chief to address a joint meeting of Congress, spoke a day after having met with Trump, where both officials hailed the increase in alliance spending.

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But Trump has in the past expressed reservations about NATO, primarily stemming from the spending issue, which in turn threatened to rock the alliance.

The New York Times reported in January that Trump had suggested withdrawing the U.S. from NATO several times in 2018, and that officials feared he could return to the threat if allied military spending continued to lag.

On Tuesday, Trump said along Stoltenberg that the relationship with NATO “has been very good” and he hailed “tremendous progress” on the question of defense spending.

But he noted that Germany, in particular, was not, according to him, “paying what they should be paying.”

"Germany is not paying their fair share," he said.

The Associated Press contributed to this report.