WASHINGTON — The Pentagon has redirected enough money to build 256 miles of barriers along the southwestern border, including 63 miles within six months, the acting defense secretary told lawmakers on Wednesday.

The remarks by the secretary, Patrick M. Shanahan, were a major step in President Trump’s effort to build his border wall without congressional authorization since he declared a national emergency and took other measures to redirect funds in February. They came the same day immigration officials reported that illegal border crossings continued to surge in April, with the highest number of monthly arrests since 2007.

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Mr. Trump’s efforts to divert funds to build the barriers are facing challenges in court, and have met fierce resistance from Democrats and some Republicans in Congress. Democrats have also expressed skepticism about a White House request for billions of dollars more in enforcement efforts at the border.