Fight Against ISIS: 'We're Not Done,' Says Gen. John Allen

— -- Just over one year after the U.S. began its mission, along with Arab allies, to launch airstrikes targeting the ISIS terror group in Iraq and Syria, the top U.S. envoy in the coalition says there is still much work to be done.

"We're not done yet," retired Gen. John Allen told ABC News' Martha Raddatz in an interview today on "This Week."

"There's much work to be done, but we've had some real successes," Allen said. "Tikrit was taken back by Iraqi security forces. Tikritis are returning home now... And in Syria -- in that place called Kobani, where we thought we would see another horrendous massacre occur, brave defenders supported that coalition, ultimately held the city."

Allen also acknowledged that the ongoing refugee crisis, unmatched in scale since World War II, is due in large part to actions of Syrian President Bashar Assad.

"Back in 2011, when the Arab Spring had such promise in many countries, it had great promise in Syria. But he chose, rather than to listen to the dissidents, he chose to attack them," Allen said. "And that created a sequence of events that has delivered us to where we are today."

Yet Allen did not express any interest in targeting Assad’s military, a move that would be a dramatic shift in policy.

"No matter what we do militarily, Syria has got to be solved at a political level," Allen said.

The humanitarian crisis has led to the flow of hundreds of thousands of refugees into Europe. The Obama administration said this week it wants to take in 10,000 Syrian refugees in 2016. But Allen notes that the U.S. needs to be cautious and that there is a potential threat that ISIS could infiltrate these refugee groups.

"I think we should watch it. We should be conscious of the potential that Daesh may attempt to embed agents within that population," Allen said, using another common name for the terror group.

But Allen was quick to say that the FBI and the Department of Homeland Security have the ability to thwart any potential threat within the refugee population.

"I have confidence that they'll work very, very hard to prevent them from getting into the country," Allen said.