WASHINGTON — Arrests and deportations of undocumented immigrants inside the United States dropped in the past year, according to government data released on Wednesday, a sharp contrast to the “millions” of deportations that President Trump promised this summer.

Immigration and Customs Enforcement agents arrested about 143,000 immigrants in the country’s interior from October 2018 to September 2019, 10 percent less than they did the previous fiscal year. That is the lowest level of arrests since Mr. Trump took office, ICE officials said Wednesday.

Deportations from the interior also declined 10 percent, to 85,958 from 95,360.

Matthew Albence, the acting director of ICE, attributed the declines in enforcement to the reallocation of resources to the border with Mexico, where a surge in asylum-seeking migrants has taxed federal resources. In May, the authorities arrested more than 144,000 migrants at the border, the highest monthly total in more than a decade.

“As a direct result of what happened at the border, we had to deploy resources which resulted in fewer arrests in the United States,” Mr. Albence said.