BRUSSELS—European leaders approved plans aimed at stepping up military spending, which officials said is partly a response to pressure by the incoming Trump administration to shoulder more responsibility for the continent’s defense.

Still, reaching final agreement on the details in coming months could be tricky. Divisions remain within the bloc about how a new defense purchasing group would work, how costs would be divided and who would be able to use the new defense capabilities that emerge.

Federica Mogherini, the European Union’s foreign-policy chief, said in an interview with The Wall Street Journal that Europe knows “we have to do more and better” on military spending.

She said the bloc’s new defense plans, which include setting up a club of EU countries to create common defense assets, would be broader than many currently expect. It could prove a “game-changer” that would strengthen trans-Atlantic military cooperation.

“This can be part of the new transactional approach with the U.S. because [President-elect Donald] Trump is on the record saying that Europeans need to do more on their responsibilities for European defense,” she said.