US Border Patrol agents report making 51,912 arrests in May, the third month in a row the number has topped 50,000.
The Trump administration uses the arrest numbers as the best gauge of whether illegal crossings are going up or down.
Border arrests fell last year to the lowest level since 1971. But since April 2017, arrests have climbed steadily.
Curbing illegal immigration is an administration priority.
Trump cited the issue as an achievement in a tweet earlier this week on his 500th day in office. But last month, he criticised the Homeland Security secretary for failing to stop rising numbers.
A spokesman for the Department of Homeland Security, Tyler Houlton, said in a statement today that reversing years of political inaction takes time.
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