President Donald Trump's top advisors were caught off guard when the president failed to reaffirm NATO's principle of mutual defense during his speech to the military alliance in Brussels.

The advisors – National Security Advisor H.R. McMaster, Secretary of Defense James Mattis, and Secretary of State Rex Tillerson – had pushed to have the language included in Trump's speech during his first overseas trip.

Key allies were looking for reassurance that the U.S. would live up to its commitments to defend fellow signatories, after a series of statements by candidate knocking the powerful alliance as expensive and obsolete.

During his speech, Trump instead lectured fellow leaders about living up to their commitments to spend 2 per cent of their GDP on defense.

President Trump lectured NATO allies for not meeting their defense commitments, but dropped language that would have reaffirmed the U.S. commitment to mutual defense

The language backing up NATO's core principle, meant to reassure allies, was included in a draft speech up until the morning it was given, Politico reported.

'There was a fully coordinated other speech everybody else had worked on,' a White House official told the publication.

“They had the right speech and it was cleared through McMaster,” said a source briefed by NSC officials after the meeting told the publication. “As late as that same morning, it was the right one.”

The top advisors – whose selection by the governing neophyte Trump helped soothe the political establishment – weren't aware of the reversal.

BLINDSIDED: U.S. Secretary of State Rex Tillerson, right, and U.S. Secretary of Defence Jim Mattis were among a trio of aides who pushed to include a reassurance of the U.S. commitment to mutual defense during President Trump's NATO speech

“They didn’t know it had been removed,” a third source said. “It was only upon delivery.”

The article floats different versions of who dealt the final blow to the statement: one is that the president nixed it himself, the other is that nationalist reemerging chief strategist Stephen Bannon and speechwriter Stephen Miller were involved.

National Security Advier H.R. McMaster at the White House in Washington, Thursday, June 1, 2017, followed by chief strategist Stephen Bannon (back)

The report said the language the advisors pushed for was still in the speech the morning it was delivered

U.S. President Donald Trump (C) walks past French President Emmanuel Macron (L) and German Chancellor Angela Merkel on his way to his spot for a family photo during the NATO summit at their new headquarters in Brussels, Belgium May 25, 2017

President Donald Trump during the North Atlantic Treaty Organization summit in Brussels

Trump's failure to deliver the line drew notice during his trip, and forced his team to explain its absence as insignificant.

White House press secretary Sean Spicer told traveling reporters after the speech that Trump is 'fully committed” to the mutual defense pact. “We’re not playing cutesy with this. I’ve seen some of the questions I’ve gotten from you guys, but there’s 100 percent commitment to Article 5,' he said.

During the same speech where he didn't mention Article 5, Trump went after allied nations for shirking their responsibilities. '

“We’re not playing cutesy with this. I’ve seen some of the questions I’ve gotten from you guys, but there’s 100 percent commitment to Article 5,' White House Press Secretary Sean Spicer told reporters when asked about Trump's speech

'NATO members must finally contribute their fair share and meet their financial obligations,' he told the world leaders who were there. 'This is not fair to the people and taxpayers of the United States. And many of these nations owe massive amounts of money from past years and not paying in those past years.'

Trump's public approval in the Gallup daily tracking poll dropped from 42 per cent on May 26, the day after the summit, to 36 percent at the end of last week.