BERLIN (AP) — American B-52 Stratofortress bombers were flying over all 30 NATO countries on Friday in an exercise the U.S. military said was meant to demonstrate the alliance's solidarity, amid growing signs of cracks.

Four of the long-range jet bombers based in Britain were flying over European members of NATO, integrating along the way with several partner nations' fighters and refueling aircraft. Two other Stratofortresses based in North Dakota were flying over Canada and the United States.

The single-day mission is part of regular flight missions that have been taking place in Europe since 2018, but is meant specifically to “demonstrate NATO solidarity, enhance readiness and provide training opportunities” by involving the airspace of every member nation, the U.S. European Command said.

It comes amid an escalating dispute between NATO nations Greece and Turkey over jurisdiction over waters in the Mediterranean, and strong-arm tactics from U.S. President Donald Trump to press NATO allies to spend more on defense that have left many of them resentful and concerned about America’s commitment.

“U.S. security commitments to the NATO Alliance remain ironclad,” said Gen. Tod Wolters, commander of U.S. European Command, in a statement. “Today’s bomber task force mission is another example of how the Alliance sustains readiness, improves interoperability and demonstrates our ability to deliver on commitments from across the Atlantic.”

He said such operations also served to emphasize the American “commitment to global security and stability.”

“By further enhancing our enduring relationships, we send a clear message to potential adversaries about our readiness to meet any global challenge,” Wolters said.