WASHINGTON — Hours after President Donald Trump departed NATO headquarters Thursday, U.S. military leaders embarked on a full-scale “damage control” operation with calls to their counterparts across Europe to reassure them that America will abide by its defense commitments in the region.

The outreach, directed by the Pentagon leadership, came after Trump threatened to reassess those commitments during a gathering with NATO allies in Brussels, according to multiple current and former diplomatic and military officials familiar with the calls.

The overall message from senior military officials in a series of phone calls to members of the North Atlantic Treaty Organization has been that U.S. military bases in their countries will remain open and American troop levels in the region will not be reduced.

The Pentagon-led effort was meant to counter Trump’s withering criticism about the security alliance that rattled European leaders.

“One thing you need in this alliance is predictability,” one diplomatic official said.

The direct conversations were aimed at “reinforcing alliance commitments,” after Trump “made it clear alliance commitments were on the table,” according to one U.S. official familiar with the discussions.

When asked at a Thursday morning press conference whether he had threatened to pull out of NATO, Trump replied, “that’s unnecessary and people have stepped up today like they’ve never stepped up before. And remember the word, $33 billion more they’re paying.”

On Wednesday, Trump asked NATO countries to raise their defense-spending threshold to 4 percent of GDP.

Speaking to reporters after the conclusion of the two-day summit, French President Emmanuel Macron denied the spending levels would reach historic highs.

“The communique is clear," Macron said. "It reaffirms a commitment to 2 percent in 2024. That is all."

That commitment was made two years before Trump was elected.

But a senior U.S. administration official described Trump as "aggressive" behind closed doors with leaders of the 29-nation alliance and argued that they need to significantly increase their defense budgets or the U.S. will conduct a "reassessment" of its commitments to NATO.

A diplomatic official said Trump warned allies that absent an increase in their defense budgets, the U.S. will "do our own thing.”

Some European leaders in the room viewed Trump’s comments as a threat.

But during a news conference Thursday, NATO Secretary General Jens Stoltenberg called talks between Trump and the other NATO leaders a “frank and open discussion” that “made NATO stronger."

"It has created a new sense of urgency,” he said, adding "a clear message from President Trump is having an impact.”

Defense Secretary James Mattis had arrived in Brussels worried that the president would exacerbate tensions there with allies, multiple officials said, and had planned to stick closely by the president during his meetings there.

But Mattis had a slew of meetings with his own counterparts during the NATO summit and was absent for some key moments. A defense official told reporters that he would be with Trump at a news conference but the defense secretary did not make it to that event.

As of early Friday, the Pentagon had not yet responded to NBC News' request for comment.

Carol E. Lee and Courtney Kube reported from Washington, and Geoff Bennett from London.