Acting Defense Secretary Patrick Shanahan has agreed to take charge of the replacement of 78 miles (125 kilometers) of fencing on the southern frontier in Arizona and Texas, the defense department said in a statement.

"The funds were drawn from a variety of sources, including cost savings, programmatic changes and revised requirements," spokesman Tom Crosson said.

"This transfer of funds will not affect military preparedness, nor impact service member benefits."

Frustrated by Congress's refusal to provide the budget he wanted to build a wall, President Donald Trump declared a national emergency on the border in February, asking to repurpose $6 billion from military funds.

In March Shanahan authorized $1 billion to construct 57 miles (92 kilometers) of 18-foot (5.5-meter) high barrier, improve roads and install lighting.

The Pentagon has been under pressure to show it is not diverting the funds from combat missions or other vital work.

Shanahan said some of the diverted money came from unspent funds in Afghanistan.

"We found ways to do this without having any impact on readiness," he said, adding that "I won't be reprogramming any more money for the border wall."