Acting U.S. Defence Secretary Patrick Shanahan has approved the transfer of $1.5 billion US to build more than 125 kilometres of barriers on the border with Mexico, a U.S. official said on Friday, including taking about $600 million in savings from an account meant for Afghan security forces.

The latest move comes on top of a March transfer of $1 billion in military money to fund President Donald Trump’s wall, something lawmakers were highly critical of.

“The funds were drawn from a variety of sources, including cost savings, programmatic changes and revised requirements, and therefore will have minimal impact on force readiness,” Shanahan said in a statement.

Shanahan said that the U.S. military had more than 4,000 service members on the border along with 19 aircraft.

A U.S. official, speaking on the condition of anonymity, said the latest transfer would include $604 million from funds for the Afghan security forces, which are struggling to hold territory against Taliban militants.

The United States had appropriated $4.9 billion in support for Afghan security forces for fiscal year 2019. The official said the money was taken from that account because they found savings in contracts.

“It is not reflective of anything related to our commitment … It took less money to meet the policy commitment than we thought,” another U.S. official said, adding that this was not the first time the United States had reprogrammed money from the account.

Patrick Shanahan has been formally nominated to succeed James Mattis, months after he took over the role in an acting capacity. (Susan Walsh/Associated Press)

The move comes as the United States is in talks with the Taliban to end the more than 17-year-old war. U.S. and Taliban negotiators wrapped up their sixth round of peace talks on Thursday with “some progress” made on a draft agreement for when foreign troops might withdraw from Afghanistan.

But there is concern that the overstretched Afghan forces could crumble if U.S. troops leave the country.

Dick Durbin, Democratic senator from Illinois, was among those Friday to criticize the redeployment of funds, pointing out that military bases in the U.S. have suffered extensive damage in the past year due to weather events.

Shanahan has yet to go through confirmation hearings for the cabinet post, over three months after Trump tapped the longtime Boeing executive to succeed James Mattis as defence secretary.

Trump formally nominated Shanahan for the post on Thursday.