The U.S. and several Middle Eastern allies struck Islamic State militants in Syria, the Pentagon said Monday night, expanding a U.S. campaign into a country that has been a haven for the extremist group.

Using Navy Tomahawk cruise missiles and Air Force fighter planes, the U.S. carried out plans for strikes against more than a dozen targets inside portions of Syria controlled by Islamic State militants, officials said.

Adm. John Kirby, the Pentagon press secretary, confirmed the strikes Monday night. […]