The Pentagon sent additional military planners to Iraq on Tuesday to figure out a way to rescue and relocate the tens of thousands of religious minorities trapped on a mountain by Islamic militants, senior U.S. officials said.

A senior U.S. official told Fox News that 130 military personnel have arrived in Irbil for the job.

The troops will work with State Department officials and USAID to develop plans to help the Yazidi people, a religious minority displaced on Sinjar Mountain.

Defense Secretary Chuck Hagel formally announced the deployment in remarks to Marines at Camp Pendleton, California.

"This is not a combat boots on the ground kind of operation," Hagel said.

The deployment comes as Defense officials openly voice doubts about the impact airstrikes alone can have, and as Kurdish forces struggle with the rescue mission.

When President Obama authorized military force last week, it was for the dual purpose of protecting American personnel and helping Kurdish forces as they try to aid members of the Yazidi minority trapped in the Sinjar mountain range. They were driven there by militants with the Islamic State (IS), and have been relying largely on international aid drops for food and water.

Officials say any relocation effort likely would involve international partners.

The planning, though, is complicated by the administration’s directive not to send ground troops. Absent that, the U.S. would have to pursue an airlift mission.

Deputy National Security Adviser Ben Rhodes told Fox News on Tuesday that both an airlift mission and an operation to create safe passage for the refugees to leave on their own are among the options being considered. This would be important, he said, to avert an act of "genocide."

One official told Fox News that even the most "Herculean effort" to lift the refugees off the mountains would take hundreds of flights and 10 days or more of constant missions.

An airlift of this sort would also come with considerable risk.

To date, the U.S. has not encountered any anti-aircraft fire, but that could change given the heavy weaponry the Islamic State has at its disposal.

Earlier Tuesday, one Iraqi helicopter crashed shortly after picking up refugees.

The consideration of such a mission comes after several days of airstrikes on the IS militants. The Pentagon currently has 250 military advisers in Iraq.

The Associated Press contributed to this report.