Pentagon rejects DC mayor's request for National Guard to assist with migrants arriving in the nation's capital

(CNN) -- The Pentagon has declined Washington, DC, Mayor Muriel Bowser's request for aid from the DC National Guard to help with migrants entering the city by bus, according to a US defense official.

Defense Secretary Lloyd Austin rejected the request because the Department of Defense has "determined providing this support would negatively impact the readiness of the DCNG and have negative effects on the organization and members," the official said.

The official said grant funding through FEMA's Emergency Food and Shelter Program is sufficient. "We understand SAMU First Response has received grant funding through FEMA's Emergency Food and Shelter Program and has indicated that sufficient EFSP funds exist at this point to provide migrant assistance."

Bowser told reporters Friday, "I have had the opportunity to scan that letter, and what I see is we may send an amended request."

"The Department of Defense highlighted a number of -- the concern about the open-ended nature of our request, and their ability to respond to it. So I want to give them some more specific parameters to better able respond to our request," she continued.

CNN previously reported that according to the office of Texas Republican Gov. Greg Abbott, more than 5,100 migrants have arrived in Washington from Texas on more than 135 buses.

The city has facilitated the help of the Federal Emergency Management Agency with a $1 million grant, the mayor said at a news conference last week.

However, city officials and non-government organizations have been increasingly concerned about the pace of arrivals over recent weeks. The mayor's office requested to convert the DC Armory, Joint Base Bolling, Fort McNair or other "suitable federal location in the National Capital Region" into a processing center for the migrants. The regional welcome center established to aid the migrants in Montgomery County, Maryland, is at capacity, according to the request.

"I've asked for the deployment of the Guard as long as we need the Guard to deal with the crisis that we expect to escalate," Bowser said last week. "The number of people crossing the border seeking asylum we expect to only go up. And we need to make sure that there is a national response, not an ad-hoc, city by city, state by state response."

Bowser also highlighted the differences in the situation in DC compared to other states.

"We need to ask the President for the use of our National Guard, which we know can be very helpful in these logistical matters," she said at the time.