BRUSSELS—A U.S. effort to enlist Europe in its pressure campaign against Iran faced a setback after officials said ministers from several European Union members will likely skip a summit organized by Washington on Iran and the Middle East.

The summit, which will be co-hosted by Poland and the U.S. and take place in Warsaw, was announced during Secretary of State Mike Pompeo’s tour of the Middle East last week. The two governments said it would focus on terror, extremism and missile proliferation in the region and threats posed...