White House: Latest ISIS execution "underscores" group's barbarity

White House: "No question" ISIS threatens U.S... 04:16

The latest execution carried out by the Islamic State of Iraq and Syria - resulting in the death of British aid worker David Haines - shows the "true colors" of the group, and stiffened the U.S. resolve to destroy them, White House Chief of Staff Denis McDonough said in an interview on CBS' "Face the Nation" Sunday.

British leaders vow justice after ISIS execut... 01:32

"We were obviously painfully aware of their barbarity and their nefariousness even before this video. But this obviously underscores it, yet again," McDonough said. "It really shows their true colors."

The Islamic State of Iraq and Syria (ISIS, also known as ISIL) previously executed two U.S. journalists, James Foley and Steven Sotloff. McDonough said the latest death "underscored our resolve to stand firm, shoulder to shoulder with the United Kingdom but also with this coalition to include of Muslim countries from the region to take this fight to ISIL, to degrade and ultimately destroy them."

President Obama delivered a speech Wednesday evening promising to strike the group "wherever they exist," and said the U.S. will expand its airstrikes from Iraq into Syria as well. Secretary of State John Kerry, who appeared in a separate interview on "Face the Nation" Sunday morning, has been traveling through the Middle East to build an international coalition of countries who will participate in the fight.

"People should not think about this effort just in terms of strikes. In fact, as some have pointed out, that alone is not going to resolve this challenge," Kerry said.

McDonough said that in response to the latest execution, people can expect "a continued strong, steady reaction against this threat" including a strong international coalition that includes other Muslim countries.

Bob Schieffer on ISIS: "This evil must be era... 01:33

But the president is also hoping Congress will approve a request to increase his ability to train and equip moderate Syrian rebels, who will be the ones fighting ISIS on the ground in the region, McDonough said.

"That's exactly what we want to do so that we can put Syrian boots on the ground to take this fight to ISIL, not have to rely on ours or somebody else's. Ultimately the President has made his decision on that. We're going to provide our unique capabilities and airstrikes and intelligence and training and then it will be up to the Syrians on that side of the border to finish the job," he said.

Asked about the danger the group poses to the U.S., McDonough said there is "no question that they are a threat to our national security."

"What we have said is that we are not aware of any credible threats to the homeland right now but we are concerned about three things in particular: The fact that they now control territory, and that gives them place to plot and plan; the fact that they are now getting not only new fighters but increased money and followers and adherence; and third, and perhaps most importantly, we are worried about the number of foreign fighters that are going into Syria to fight," McDonough said.

Experts worry that foreign fighters from Western Europe and the U.S. have the necessary passports to re-enter their home countries with ease and could carry out attacks there.

"We're going to make sure that does not happen by sharing information with our partners by keeping the pressure on our partners to not allow people in and out of Syria and to make sure that - as the president will do later this month up in the United Nations - that we have the tools to stop that kind of travel," McDonough said.

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