President Trump foreshadowed the tension that is sure to erupt at next month's NATO summit.

While at the G7 summit in Canada, Trump complained to American allies that "NATO is as bad as NAFTA," Axios reported Thursday.

The statement was worrisome for some world leaders, who plan to discuss the military alliance with Trump in Brussels on July 11. Trump reportedly said "it will be an interesting summit," and drew a parallel between NATO and NAFTA by calling them both "much too costly for the U.S."

Trump has consistently bashed NAFTA, calling the trade agreement a "cash cow" for other nations and threatening to stop cooperating unless Canada and Mexico chip in more substantially. Drawing an equivalence between the two agreements has world leaders worried that Trump will seriously undermine efforts at the NATO summit, reports Axios.

The White House didn't deny that Trump said as much to his global counterparts, but said that he "engaged in a constructive dialogue" while at G7. World leaders attending the NATO summit are bracing themselves for a rocky couple of days, and hoping Trump will opt for a more diplomatic tone when the time comes. Read more at Axios. Summer Meza