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 by proxy groups, activities Washington has accused Iran of engaging in or promoting.

Mr. Pompeo said the meeting would bring together dozens of countries to “focus on Middle East stability…and that includes an important element of making sure that Iran is not a destabilizing influence.”

The choice of Poland, a member of the EU, to co-host the summit was seen by some European diplomats as an effort to split the EU over Iran.

The bloc is a strong supporter of the 2015 Iranian nuclear deal and is seeking to help retain economic ties with Tehran despite renewed U.S. sanctions on Iran following President Trump’s decision in May to withdraw from the nuclear deal.

The U.S. withdrawal, however, has driven many European companies to leave Iran for fear of being targeted by U.S. sanctions.

On Thursday, a senior EU official said EU foreign policy chief Federica Mogherini wouldn’t attend the Warsaw gathering because of a prior engagement. Ms. Mogherini has spearheaded EU efforts to sustain the 2015 deal, which offered Iran relief from sanctions in exchange for steps to curb its nuclear program.

The senior EU official said the bloc remains unclear on what vision of peace and stability in the Middle East the Feb. 13-14 summit will aim to promote. There is “a lot of uncertainty about participation of many other EU member states at ministerial level,” the official said.

European diplomats said in recent days that France is unlikely to send its foreign minister. Britain and Germany haven’t decided who will represent them. Luxembourg’s foreign minister said he would miss the event because of a prior arrangement.

One European diplomat said the bloc wouldn’t be “joining an anti-Iran coalition.”

The Iranian foreign ministry summoned Poland’s ambassador to protest the summit on Sunday.

The “Polish government can’t wash the shame: while Iran saved Poles in WWII, it now hosts desperate anti-Iran circus,” Iran’s Foreign Minister Javad Zarif wrote on Twitter.

Polish officials have been trying to play down the Iran focus of the summit. Poland’s foreign minister has asked to formally raise the meeting at a regular EU foreign-affairs meeting on Monday, EU officials said.