At the G7 summit earlier this month, the US president claimed the military group were as bad as his often hated North American Free Trade Agreement (NAFTA).
The NAFTA is a trilateral agreement signed by Mexico, Canada, and the US, designed to make trade between the three countries easier and quicker.
According to Axios news, Trump made the inflammatory comments to other G7 leaders at Canada’s meeting.
He allegedly said: “It will be an interesting summit.
“NATO is as bad as NAFTA.
“It’s too costly for the US.”
The comments could cause concern ahead of the NATO summit in Brussels next month.
Other members will be reluctant for Trump to cut funding or US troops from the program.
Number 10 wanted to calm any rumours the President may be withdrawing from the agreement.
A spokesman for Theresa May said: “The President has made clear his commitment to NATO numerous times.
“Including when the Prime Minister met him last year and we believe his words were backed up by action and of course they will be discussing all of that next week.”
Relations between Trump and Europe have been tense in recent weeks.
At his rally in North Dakota on Wednesday evening, the President said the EU was simply there to take advantage of the US and raid their “piggybank”.
During his speech to the 6,000-strong crowd Trump accused the Brussels bloc of taking advantage of US trade to the value of £115 billion.
He said: “We love the countries of the European Union.
“But the European Union, of course, was set up to take advantage of the United States, to attack our piggy bank.
“We can’t let that happen.”