"I think the one area where we obviously are very attentive now is the expansion of Daesh beyond the region," Gen. John Allen said in an interview with CNN on Thursday, using another name for ISIS, or ISIL. "We're watching that very closely."
Speaking with CNN's Elise Labott on his last day in office, Allen, presidential envoy for the anti-ISIS coalition, said the United States is watching ISIS affiliates in Libya, the Sinai Peninsula in Egypt, the North Caucasus along the Russian border, Yemen and the Khorasan across Afghanistan and Pakistan.
"We have turned a lot of attention to that over the last several months," Allen said. "When we committed ourselves ultimately to deal with this initially, it was an ISIL that was largely contained within Iraq and Syria. In the aftermath of that containment or that commitment, other organizations around the world put their hands in the air to join the so-called caliphate."
Allen said that as the fight against ISIS has grown more prolonged, the U.S. has been learning that it cannot combat terror groups -- which Allen labeled as symptoms -- without addressing underlying conditions that help their spread.
That understanding has driven America's pursuit of partners in the region, Allen said.
"Nobody understands this region better than the people in this region, and by working closely with them to try to get at these underlying conditions," he said. "If we don't get to the left of those symptoms and try to solve these underlying circumstances, working collaboratively with those who are in the region ... then we're going to be condemned to fight forever."
On the topic of Syria, Allen threw cold water on the idea of a no-fly zone, despite several presidential candidates advocating for the move.
Allen said the United States has examined establishing a no-fly zone over the conflict-torn nation, but said "now is not the time to seriously consider it" if conditions aren't suitable.
While he didn't specify the exact nature of the unsuitable conditions and whether they applied at this point, he gave no indication that the United States thought it made sense to proceed with a no-fly zone in the immediate future.
"It's not just a no-fly zone in a place or ... whether it's in the air or on the ground. It's also a matter of timing as well," Allen said.
"The intricacies and the complexities and the cost, frankly, in terms of resources, additional resources, of a no-fly zone or a safe zone are not insignificant," Allen said. "And the question then becomes: What do we want to accomplish with them? And if the conditions are not suitable right now for what we want might want to accomplish, then now is not the time to seriously consider it."
Candidates ranging from Democratic former Secretary of State Hillary Clinton to Republican neurosurgeon Ben Carson have called for the United States to establish a no-fly zone in Syria to try to stem the bloodshed and give U.S.-supported forces a chance to succeed in the fight against ISIS in the region.
Allen also took stock of the situation in the fight against ISIS in Iraq, where on Thursday the Kurdish military force fought to take back the northern Iraqi town of Sinjar with support from coalition airstrikes.
The operation includes up to 7,500 Peshmergas -- the Kurdish military force -- who are attacking the city from three sides to take control of supply routes, according to the Kurdish Region Security Council.
"As you recall about this time last year, we were all facing the horror of what was happening in the vicinity of Sinjar to the Yazidi people. A year later, Kurdish forces, some number of thousands of them, have launched into the attack to push Daesh (ISIS) out of that area," Allen said, referring to the massacre last year of thousands of members of the Yazidi minority who lived in the region as tens of thousands more sought refuge on Mount Sinjar from ISIS' onslaught.
Allen said the Kurdish forces were also fighting to cut off what's called Route 47, the artery that passes through the town linking the Iraqi city of Mosul with cities it holds in Syria.
"That's a very important development, and it continues to indicate how with coalition air power, American advice, coalition advice, partners like the Peshmerga and other elements within those formations are able to make real ground," Allen added.
But Labott pushed back on why, with the Kurds such strong partners in the fight against ISIS, the United States is not directly arming them.
Allen said the Peshmerga are being supported and armed by 14 countries, but having that support flow through Iraq is part of the design of American intervention in the region.
"One of the reasons we came to Iraq, one of the reasons we committed ourselves, was to restore the territorial integrity of Iraq and the sovereignty of the Iraqi government over all of Iraq," he said.
"While we may not be directly providing them assistance, the idea that the assistance flows through Baghdad ... has both provided for the support to the Kurds but also has reinforced the nature of the sovereignty of the Iraqi government," he continued. "That's inherently the reason that we're operating this way."
President Barack Obama tapped Allen, the retired four-star Marine general and former commander of American forces in Afghanistan, to coordinate the international coalition to battle ISIS.
Since September of 2014, he has worked to build a 63-nation coalition and set up the infrastructure for confronting the terror group, including efforts to crack down on ISIS finances, stop the flow of fighters and countering the group's extremist ideology. He is also credited with negotiating U.S. access to Turkish airbases for strikes in Syria, which has helped the United States increase its air campaign and support Syrian rebel and Kurd offensives against the ISIS stronghold in Raqqa.
During Allen's time in the military, he served as deputy commander in Iraq's Anbar province and as deputy commander for U.S. Central Command. He was also the commander of NATO-led forces in Afghanistan from 2011 to 2013. There, he worked to build relationships with Sunni tribal leaders in a movement, known as the Anbar Awakening, credited with helping turn the tide against al Qaeda in Iraq.
Secretary of State John Kerry later appointed him to develop a security plan for Israel and the West Bank during negotiations between Israelis and Palestinians over the possibility of a future Palestinian state, which eventually collapsed.
Allen said he is looking forward to down time but also left the door open to further service.
"I've told the President of the United States that I will always serve him -- just as I did with John Kerry. I'll come off the bench if they need the help," he said.