The Pentagon is preparing to send surveillance aircraft, including drones, into Syrian airspace to gather intelligence on Islamist targets, laying the groundwork for a possible expansion of the limited U.S. military air campaign beyond Iraq, senior U.S. officials said.

The decision amounts to an acknowledgment that U.S. intelligence-collection efforts must be expanded to provide a better picture of the threat posed by the group calling itself the Islamic State, which holds large swaths of Syrian and Iraqi territory. It is...