Non-border immigration arrests in the United States fell by 10 percent in the year to September from a year earlier, the latest U.S. data shows, a drop that the head of Immigration and Customs Enforcement (ICE) said was partly due to the diversion of the agency's resources to the border.

Efforts to arrest immigrants inside the United States were hampered by a surge of mostly Central American families and children arriving at the U.S.-Mexico border earlier this year, which diverted the agency's resources, said ICE acting Director Matthew Albence.

He said 350 of the agency's more than 6,000 Enforcement and Removal Operations personnel were reassigned to deal with the border crisis at various times during the last year. Overall, ICE has more than 20,000 employees.

The acting Director said the arrival of hundreds of thousands of migrants meant an increased workload in detention centers and in the field.

"Simply put, there are only so many resources to go around," he said. "We've had some places where, for months at a time, fugitive operations teams were shut down."

Albence also attributed the lower arrest totals to a lack of cooperation from "sanctuary" jurisdictions that do not honor the agency's requests to hold immigrants booked into local jails. "It's compounded by jurisdictions not cooperating with us," he said.

President Donald Trump has made immigration enforcement a central focus of his presidency and reelection campaign. He tweeted in June that immigration authorities would begin to deport "millions of illegal aliens" without legal status.

U.S. immigration officers arrested approximately 143,000 immigrants in fiscal 2019, which ended September 30, down from nearly 159,000 arrests a year earlier, according to the report.