Pentagon officials on Tuesday described airstrikes in Syria against the Islamic State in Iraq and Syria (ISIS) as successful, but cautioned that they were just the beginning phase of a campaign that could last years.
"I would think of it in terms of years," he added about the length of the expected campaign against ISIS.
Mayville, who is the director of operations for the Joint Chiefs of Staff, cautioned that ISIS fighters would adapt to the strikes, and attempt to melt into urban areas where the strikes could become less effective without ground troops.
"We have seen evidence that they're already doing that. We've seen that now, as a result of the air campaign thus far in Iraq," Mayville said.
"They are a learning organization, and they will adapt to what we've done ... and seek to address their shortfalls and gaps against our air campaign in the coming weeks," he said.
He reiterated that there would be no U.S. troops on the ground in Syria to direct airstrikes against hard-to-find targets.
"We have not put, and we will not put ground forces into Syria," he said.
At the same time, he acknowledged that ground forces were preferred when there were concerns about collateral damage or about precision in a closed or urban environment where there is a convergence of forces.
"There's obviously a desire to put something on the ground," he said. "We've been able to provide air support without putting forces forward, and I think we will continue to look at how we can do that as we move forward."
The Pentagon conducted 14 strikes in Syria on Monday evening, hitting ISIS targets as well as an al Qaeda affiliate known as the Khorasan group, who officials say were in the "final stages" of executing an attack on Western targets, including potentially the U.S. homeland.
There were three waves of attacks in Syria, carried out by the U.S. and Jordan, the United Arab Emirates, Bahrain, and Saudi Arabia, with Qatar in a supporting role.
In the first wave, two guided missile destroyers, the USS Arleigh Burke in the Red Sea and the USS Philippine Sea in the northern Persian Gulf launched more than 40 Tomahawk cruise missiles at targets in eastern and northern Syria.
Those strikes hit near Aleppo and Raqaa, with the majority targeting Khorasan Group compounds, manufacturing workshops and training camps.
A second wave consisted of F-22 Raptors — making their combat debut — F-15 Strike Eagles, F-16s, B-1 bombers and drones launched from regional bases against targets in northern Syria, including ISIS headquarters, training-camp barracks and combat vehicles.
The final wave consisted of F-18s from the USS George H.W. Bush aircraft carrier in the northern Persian Gulf and regionally based U.S. F-16s jets, attacking ISIS training camps and combat vehicles in eastern Syria.
Officials said coalition partners participated in the second and third waves, "supporting with a range of combat capabilities including combat air patrols and actual strikes."
"The preponderance of the force came from U.S. platforms," Mayville said.
The administration has touted the role of allies, with President Obama saying Tuesday that the U.S. was not acting alone.
The U.S. has also been striking ISIS targets in Iraq since early August. Pentagon officials said strikes against ISIS in Iraq have been successful so far, but that more time is needed to assess their overall effect.
"The most important thing is to create some space for the Iraqi security forces to reorganize and replace leadership...and to allow them to get on the offensive," Mayville said.
Mayville said he did not want to reveal too much information about future strikes, since it was an ongoing campaign, but said to expect more like those conducted on Monday.
"You are seeing the beginnings of a sustained campaign, and strikes like this in the future can be expected," he said.
This story was updated at 2:44 p.m.