President Trump requested $4.5 billion from Congress in emergency funding to address the large numbers of migrants attempting to cross the southern border into the U.S., a problem that Mr. Trump has frequently described as a "crisis." The request does not include money for a border wall, which congressional Democrats have refused to fund

The White House is asking for $3.3 billion that would among other things, expand the shelter capacity for unaccompanied migrant children, provide food, care and transportation for those in custody, and also fund processing centers at the border.

The letter from the administration to House Speaker Nancy Pelosi notes that the number of unaccompanied children whose care is charged to the Department of Health and Human Services (HHS) increased by almost 50 percent over the past year.

"This shift in demographics has stressed a system mainly designed for single adults," Acting Office of Management and Budget Director Russell Vought said in the letter. Vought also faulted Mexico for not doing more to stem the tide of migrants passing through from Central America to the U.S.

The letter also requests $1.1 billion for operations support on the border, which would be used to address volunteer surge requirements, buy more detention beds, and pay for investigations to counter human smuggling and trafficking. Mr. Trump has previously sent military personnel down to the border, although these forces do not interact directly with migrants, but provide logistical support and assist in repairing and building fencing.

HHS also asked for $178 million to upgrade information technology systems and fund law enforcement pay adjustments to manage the continuing surge. 

The agency said that if Congress doesn't provide the additional funding, and the surge continues, HHS might have to divert resources from other programs "that serve vulnerable populations - such as refugees and victims of trafficking and torture," the letter said.

The letter comes over two months after Mr. Trump declared a national emergency at the border in order to obtain funds to build the border wall without congressional approval. An effort to block Mr. Trump's national emergency by Congress was vetoed.

Mr. Trump has repeatedly criticized congressional Democrats as being unwilling to work with Republicans to fix the immigration system.

"President Trump is committed to working with the Congress to enact legislation that will give our Nation a secure, coherent, rational, and merit-based immigration system," Vought concluded in his letter.

Separately, CBS News confirmed Wednesday that the Department of Homeland Security will launch a rapid DNA testing program at the border. The goal will be to identify and prosecute people posing as families in an effort to target human smuggling. This will be a pilot program which will involve a cheek swab and can provide results in about 90 minutes.

Jeff Pegues contributed to this report