The U.S.-led campaign against the Islamic State (ISIS) is having an effect on gains the terrorist militants have made in Iraq and Syria, Pentagon spokesman Rear Adm. John Kirby told MSNBC's "Morning Joe."

In Iraq, the Islamic militants were "largely on the defensive now," and the airstrike campaign in Syria has "had an impact on them there as well," Kirby said Friday.

"We think the strategy is working in Iraq and in Syria against ISIL," Kirby said, using another common name for the Islamic State. "Their territory [in Iraq] has actually shrunk, and they're trying to preserve what kind of gains that they've been able to make, and not really going out on the offense."

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The campaign was an "Iraq first strategy," but not an "Iraq-only strategy," Kirby said, adding the effort there was about "taking care of ISIL in Iraq where they're operating and maneuvering."

The strategy in Syria was aimed at "denying them sanctuary and safe haven," and focused on "their ability to finance themselves, to train, to equip, to resource."

"We've hit some of their oil refineries [in Syria]. We've hit their crude oil collection points. We've taken away millions of dollars of revenue that they now no longer have their hands on. And, we've hit training camps. We've hit command and control centers," he said.

The "whole campaign purpose" in Syria was not attacking the regime of President Bashar al-Assad, but targeted "going after this particular terrorist network and their ability to operate to sustain themselves," he said.

Rather than using the military to defeat ISIS, Kirby said the emphasis was on supporting "good governance" to prevent young men to be drawn to the militants' "virulent ideology."

Iraq now had a "unity government" but, he said, in Syria "you've got the Assad regime, where there is no good governance."

It was going to take a "real concerted effort from an interagency perspective here in the United States, as well as internationally" to monitor people seeking to travel to Syria with a desire to train alongside ISIS militants, Kirby said.

"It has to be law enforcement agencies, intelligence agencies across the spectrum, sharing as much information as possible. It's a heavy lift. It's going to be very, very hard," he said.

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