Story highlights Washington is looking at options for evacuating Yazidi refugees

One focus is an air operation, but a ground one is also being discussed

Air operation would require some U.S. troops on the ground, official says

Additional U.S. military advisers arrive in northern Iraq to assess the situation there

Additional U.S. military advisers are now on the ground in northern Iraq with the Obama administration considering a possible air evacuation of thousands of minority Yazidis under threat from Islamic State militants and trapped on Mount Sinjar.

President Barack Obama ordered nearly 130 advisers to Irbil to assess the situation, and it's now likely that a small number of them will travel to the mountain area to get a first-hand look at what might be possible.

The new group comprised of Marines and special forces adds to the hundreds of other American advisers already in the country advising Iraqi troops in their fight against Sunni militants from the Islamic State, an al Qaeda off-shoot formally known as ISIS.

The group has waged a brutal campaign while seizing large areas of territory this year in Iraq. It's aiming to establish a caliphate.

Hundreds of thousands of Iraq's ethnic and religious minorities have fled ISIS fighters. Nowhere is the crisis more evident than the Sinjar Mountains, where an estimated 40,000 minority Yazidis are hiding.

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The plight of the Yazidis, coupled with the ISIS assault against Iraq's semiautonomous Kurdish region, prompted the United States to begin targeted airstrikes over the past several days. The aim, according to Obama, is to help protect U.S. personnel in the area and to destroy ISIS positions around the mountains to ease the threat to minority groups.

Air evacuation option

Obama has also authorized humanitarian airdrops to help the Yazidis, and is now weighing an air evacuation, a U.S. official told CNN on Wednesday.

Focusing on a possible air option -- rather than a ground one -- is due to initial indications that going by land would take too long would up the risk to the Yazidis, according to the official who was not authorized to speak publicly on the matter and asked not to be named.

In addition, many of the stranded Yazidis are in poor medical condition to make a lengthy journey, the official said. However, American officials did say that a ground option is still under consideration as well.

If an air operation is proposed and approved by Obama, it would require putting U.S. ground troops both on the mountain and at a nearby airfield, the official said.

The operation will be characterized as a short-term humanitarian mission, not a combat one. But U.S. troops would have the right to defend themselves against any ISIS attacks.

As part of an air evacuation, American forces would establish a security corridor around the area to try to provide airspace for helicopters and other aircraft to pick up people.

The newly arrived advisers would form the beginning of a joint task force to carry out such an operation, the official said.

They would work to develop procedures for rescuing and then processing people taken off the mountain. But there are still several unresolved questions -- such as, where will the Yazidis be flown to?

Asked about a potential evacuation operation on CNN's "New Day," Pentagon spokesman Rear Adm. John Kirby said he didn't "want to get too far out in front" of any potential scenarios.

"We shouldn't be jumping to a conclusion right now that there is or won't be a rescue operation in particular," he said.