Story highlights ISIS is "a cancer" that can spread into Europe and the U.S., Sen. John McCain says

"This is beyond anything we have seen," Defense Secretary Chuck Hagel says

Dempsey says ISIS can't be defeated without addressing its presence in Syria

U.S. Central Command has conducted 51 airstrikes near the Mosul Dam

America's top defense officials left open the possibility of targeting ISIS fighters in Syria, saying during a news briefing Thursday that it was not enough to just hit the extremist group in Iraq.

Defense Secretary Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, stopped short of calling for U.S. military action in eastern Syria, an ISIS stronghold.

"Can they be defeated without addressing that part of the organization that resides in Syria? The answer is no," Dempsey said during the briefing at the Pentagon.

Repeatedly pushed by reporters about whether that meant operations against ISIS in Syria, Hagel said, "We're looking at all options."

While it's unclear what those options may be, Hagel said the United States is "very clear-eyed" about ISIS, which calls itself the Islamic State but also is known as the Islamic State in Iraq and Syria.

"They are beyond just a terrorist group. They marry ideology, a sophistication of strategic and tactical military prowess," Hagel said.

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"This is beyond anything we have seen, and we must prepare for everything. And the only way you do that is that you take a cold, steely hard look at it and get ready."

Speaking at a town hall meeting in Arizona, Republican Sen. John McCain, who has advocated for strikes against ISIS in Syria, said the extremists were "an enemy that must be defeated. Not stopped, but defeated."

"It is a cancer which has spread throughout that region and can spread into Europe and into the United States of America," McCain said. His phrase echoed the term President Barack Obama used Wednesday to describe the terrorist group.

Hagel offered his condolences to the family of American journalist James Foley, whose execution at the hands of ISIS was captured on video and posted online this week as a warning to the United States.

The group threatened the life of another American hostage, believed to be journalist Steven Sotloff, if the United States did not end its airstrikes in Iraq.

The threat has done little to curb U.S. military operations in Iraq. On Thursday, American warplanes pounded ISIS targets near Mosul Dam, where Iraqi forces have been battling to retake control of the area.

A total of six airstrikes hit ISIS positions, according to the Defense Department. The strikes destroyed or damaged three ISIS Humvees, one ISIS vehicle and multiple locations where improvised explosives had been placed, the U.S. military said.

Obama ordered targeted airstrikes in Iraq this month to protect U.S. personnel and facilities as well as minorities being brutalized by ISIS.

Since August 8, the U.S. military has carried out 90 airstrikes, 57 of them in support of Iraqi forces near the Mosul Dam, according to the Defense Department.

Mosul Dam is the largest in the country, and Iraqi and U.S. officials fear that a breach in the dam would threaten the lives of millions of Iraqis who live downstream in Mosul and Baghdad.

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U.S. airstrikes played a role helping Kurdish and Iraqi forces retake control of the dam, pushing back ISIS militants who seized it.

With the dam clear of ISIS fighters, Iraqi forces are expanding their area of control, the Defense Department said.

The fighting and efforts by ISIS militants to take over towns in northern Iraq have forced as many as 1.2 million Iraqis from their homes so far this year, according to UNHCR estimates.

On Thursday, Kurdistan's regional government praised the assistance provided by the United States and the international community.

"We are grateful to the men and women of the American armed forces who have carried out these decisive attacks against the terrorist forces of ISIS," said a statement from the regional government.

Nechirvan Barzani, the prime minister of the regional government, described the beheading of Foley as "a horrendous act committed with utter disregard for human life."

"Like Mr. Foley, several hundred innocent members of the Yazidi and Christian communities have been killed in the same manner," Barzani said in a statement Thursday. "There is no way to overstate the extent of this inhumanity, and it illustrates the true, dark nature of the terrorists that we are confronting."

Also Thursday, a third French aid flight headed to Irbil, according to the French foreign ministry. The aid includes 21 metric tons of food, tents, medical supplies and other basic necessities. France has delivered 58 metric tons of aid since it started its humanitarian flights on August 15, the ministry said.