On Sunday, President Donald Trump delivered his most explicit warning yet to the North Atlantic Treaty Organization (NATO): Align with the U.S. to defend the Strait of Hormuz or face a “very bad” future.

“It’s only right that those who benefit from the strait help ensure nothing harmful occurs there,” Trump told The Financial Times in a Sunday interview. “If there’s no response—or a negative one—I believe it will be very damaging for NATO’s future.”

Trump repeated those comments during a Sunday night press gaggle on Air Force One, as he returned to Washington, D.C., from a weekend at Mar-a-Lago, saying it would “be good to have other countries police that with us, and we’ll help—we’ll provide military support.”

“Remember, in many cases with NATO countries, we’re always there for NATO,” Trump told reporters, pointing to “helping them with Ukraine” even though “honestly, it doesn’t affect us.”

“But we’ve supported them,” he added, echoing remarks he made to the United Nations General Assembly last fall and questioning whether NATO will “always be there for us.”

Trump is seeking assistance from NATO allies to secure oil tanker traffic through the Strait of Hormuz for the global community. Trump administration officials have consistently stressed amid tensions over the strait that under Trump, the U.S. is a net oil exporter and sources only a small fraction of its oil from the Middle East—unlike other nations, including NATO allies.

“It would be interesting to see which country wouldn’t help with such a small effort—just keeping the Strait open—and that’s a minor request by comparison,” Trump told Air Force One reporters. “It’s small because Iran has very little military capability.”

Trump remained hopeful that NATO allies will eventually join the effort.

“We’re talking to other countries about working with us on policing the strait, and I think we’re getting a positive response,” Trump told AF1 reporters. “If we do, that’s great—and if we don’t, that’s great too.”

NATO has long been a point of friction for Trump, who repeatedly pressed member nations to meet even the 2% Gross Domestic Product (GDP) defense spending threshold during his first administration. Current U.S. Ambassador to NATO Matt Whitaker—appointed by Trump—has praised this second administration for securing NATO’s commitment to 5% GDP in defense spending.