WATCH: Defense Sec. Lloyd Austin says US doesn’t ‘have the capability’ to get all Americans out of Afghanistan

There's "a distinction between extracting someone in an extremist condition or circumstance versus going out and collecting up large numbers of American citizens," Austin said.

Austin fielded a question from a reporter, "Do you have the capability to go out and collect Americans?"

"We do not have the capability to go out and collect large numbers of people," Austin said.

This as the US State Department released a memo stating that "The United States Government cannot ensure safe passage to the airport" for American citizens.

"U.S. citizens, U.S. LPRs, and their spouses and children (under age 21) should proceed to Hamid Karzai International Airport (HKIA) as soon as possible if you judge it safe to do so.

"The security situation in Kabul continues to change quickly, including at the airport. U.S. citizens seeking assistance to depart the country should complete this Repatriation Assistance Request for each traveler in their group. Please do so as soon as possible. Please do so only once. You must complete this form even if you’ve previously submitted your information to the U.S. Embassy in Kabul by another means. This form is the only way to communicate interest in flight options. We will notify you directly by email based on your registration as soon as departure options become available.

"Do not call the U.S. Embassy in Kabul for details or updates about the flight. Please note that gates may change on a daily basis and that we will provide updates as necessary via email. The United States Government cannot ensure safe passage to the airport."

The situation in Afghanistan, which saw the US-backed government fall to terrorist Taliban fighters on Sunday, is changing minute by minute. The US military currently holds tenuous control of the Kabul airport. However, the present difficulties surrounds getting US citizens and their Afghan allies to the airport quickly and safely so that they can be then flown out.

Austin said that securing the airport and keeping it secure is the number-one priority right now.

Current estimates put the number of Americans still stranded in Afghan territory at approximately 14,000 people, although nobody is absolutely sure at this point.