The Trump administration is asking Congress for an additional $4.5 billion in emergency spending for border security as the administration contends with a surge of Central American migrants at the southern border.

That's according to two people familiar with the request who spoke to The Associated Press on condition of anonymity because they were not authorised to speak publicly before a formal announcement.

A summary of the request obtained by the AP says the White House wants $3.3 billion for humanitarian aid to increase shelter capacity for unaccompanied migrant children and the feeding and care of families.

An additional $1.1 billion would go toward operational support, including personnel expenses, detention beds, transportation and investigative work on smuggling. And the final $178 million would be used for mission support, including technology upgrades.

“DHS projects it will exhaust resources well before the end of the fiscal year,” reads the formal request letter to Congress, also obtained by the AP. “Without additional resources, the safety and well-being of law enforcement personnel and migrants are at substantial risk.”

It also says the Department of Health and Human Services, which manages the care of migrant children who arrive alone or who are separated from their parents by DHS under certain circumstances, will exhaust its resources by June.

Acting Homeland Security Secretary Kevin McAleenan said on Tuesday during a congressional hearing that the department was running out of money amid a spike in migrants crossing the southern border.

He told a House panel the money would be used for temporary and semi-permanent facilities to process families and children and increase detention, though he didn't specify a figure then.

Nearly 100,000 migrants crossed the border in March, a 12-year high.

Customs and Border Protection on Tuesday encountered its largest group to date: 424 people in rural New Mexico.