A U.S. Border Patrol agent searches an undocumented migrant who illegally crossed the Rio Grande in Los Ebanos, Texas, April 6, 2019.

U.S. Customs and Border Protection arrested nearly 51,000 migrants in August at the southern border, a sharp decline compared with the previous month, a top Trump administration official said Monday.

"Trump is making it clear: if you come to the United States illegally, you will be removed," said Mark Morgan, acting commissioner of Customs and Border Protection, in a press briefing. "The president has made it very clear to use every tool available to him to address this crisis."

The CBP and U.S. Army Corps of Engineers have also constructed 65 miles of border wall, and plan to build the rest of it by the end of 2020 to deter migrants at the border, Morgan said.

"Every single mile of wall is built, this country is more safe," he said.

President Donald Trump has made reducing illegal immigration a top priority of his administration and reelection campaign. The latest figures indicate his immigration crackdown may be reducing the number of migrants apprehended at the southern border.

Morgan added the August border apprehension numbers reflect a 56% reduction when Border Patrol arrested nearly 133,000 migrants in May. Arrests are also down nearly 30% from close to 72,000 in July.

The White House is pressing Congress to provide more money to build a border wall. The Supreme Court in July allowed the administration access to $2.5 billion from a Pentagon counterdrug fund for construction.

Last week, the Defense Department announced it would use $3.6 billion in military construction funds to pay for border projects in California, Arizona, New Mexico and Texas.