U.S. Immigration and Customs Enforcement (ICE) agents look on as an undocumented man is received by a Mexican immigration agent at a removal gate of the U.S.-Mexico border in San Diego, California, U.S., on Thursday, Feb. 26, 2015. The U.S. Department of Homeland Security is nearing a partial shutdown as the agency's funding is set to expire Friday -- something Senate Majority Leader Mitch McConnell had said wouldn't happen on his watch. Photographer: David Maung/Bloomberg via Getty Images
CNN  — 

US immigration arrests are down compared with last year, as illegal migrant crossings spike at the southern border and Immigration and Customs Enforcement has had to shift resources to the deal with the influx.

The agency has redirected “countless” resources and personnel in response to the “explosion at the border,” said Nathalie Asher, ICE acting executive associate director for enforcement and removal operations, on Thursday.

“Our interior arrests have been affected because I’ve had to redirect to the priority, as a nation, in addressing what has been occurring and continues to occur at an alarming rate at the border,” she added.

Total arrests are down 12% compared with last year, according to data released by ICE on Thursday.

ICE, the agency responsible for enforcing immigration law in the interior of the US, arrested 34,546 undocumented immigrants in the first quarter of fiscal year 2019.

During the same time period last year, the agency made 39,328 arrests.

“There are less fugitive operations and less criminal alien arrests being done in the interior of the country,” a Department of Homeland Security official told CNN last month.

The border surge has also caused a huge increase in releases of migrant families from ICE custody into the interior of the country.

From December 21 to March 20, ICE released around 107,000 family members into the interior of the US.

“A staggering number,” Asher said.

In just one day in the Rio Grande Valley, Asher said, ICE released over 1,000 individuals.

While Asher did not have a comparison number readily available, she said, “I think we can all agree it was nowhere near this number.”

Earlier this week, US Border Patrol, which polices the US land borders, took the very rare step of releasing migrant families directly from its custody with notices to appear in court – something that hasn’t been done since 1998, according to a DHS official.

Generally, migrants are transferred from Border Patrol custody to ICE for further processing and release or further detention.

In February, Border Patrol arrested 66,450 migrants illegally crossing the southern border, up nearly 150% from 26,666 arrests during the same month last year.

Homeland Security Secretary Kirstjen Nielsen, speaking from McAllen, Texas, on Thursday, acknowledged the ongoing releases of migrant families from Border Patrol custody but denied that it was a “catch-and-release” policy.

“It’s not a protocol and there’s no reintroduction of catch-and-release,” she said. “But we are out of detention space.”

ICE deportations are up almost 10% from the same time last year – primarily due to the increase in border arrests, said Asher.

The agency deported 66,549 undocumented immigrants in the first quarter of this year, up from 60,572 during the same period last year.

However, when asked why deportations are down from peak years, Asher said more people are “claiming fear of return to their home countries,” slowing down the process.

And they are getting caught up in prolonged court cases with significant backlogs, she said.

Among those entering ICE custody in the first quarter of the year, 95,494 were apprehended by Customs and Border Protection. Only 33,941 were arrested by ICE.

CNN’s Priscilla Alvarez contributed to this report.