Trump's new acting Pentagon chief just extended the mission for US troops at the US-Mexico border until the end of September

US Air Force photo by Airman First Class Daniel A. Hernandez

Army engineers install concertina wire Nov. 5, 2018, on the Anzalduas International Bridge, Texas.U.S Northern Command is providing military support to the Department of Homeland Security and U.S. Customs and Border Protection to secure the southern border of the United States.

• Active-duty troops deployed to the US-Mexico border will not be leaving anytime soon, the US Department of Defense announced Monday evening.
• The border mission, initially expected to wrap up on Dec. 15 of last year, has been extended to Sept. 30.
• The cost of the deployment is expected to hit $132 million by the end of this month.

US troops deployed to the US-Mexico border will remain there until at least the end of September, the Pentagon revealed in an emailed statement Monday evening.

Acting Secretary of Defense Patrick Shanahan, who took over for former Secretary of Defense James Mattis at the start of the year, has approved Department of Defense assistance to the Department of Homeland Security through September 30, 2019.

The decision was made in response to a DHS request submitted in late December.

The initial deployment, which began in October as "Operation Faithful Patriot" (since renamed "border support"), was expected to end on December 15, 2018. The mission was first extended until the end of January.

Thousands of active-duty troops, nearly six thousand at its peak, were sent to positions in California, Texas, and Arizona to harden points of entry, laying miles and miles of concertina wire. The number of troops at the southern border, where thousands of Central American migrants wait in hopes of entering the US, has dropped significantly since the operation began.

Read more: The military's mission at the US-Mexico border will no longer be called Operation Faithful Patriot

DoD is reportedly transitioning the support provided from securing ports of entry to mobile surveillance and detection activities. Troops will offer aviation support, among other services.

Shanahan has reportedly also given his approval for deployed troops to put up another 115 miles of razor wire between ports of entry to limit illegal crossings, according to ABC News.

The extension of the border mission was expected following a recent Cabinet meeting. "We're doing additional planning to strengthen the support that we're providing to Kirstjen and her team," Shanahan said, making a reference to Homeland Security Secretary Kirstjen Nielsen, Military.com reported earlier this month.

"We've been very, very closely coupled with Kirstjen," he added, "The collaboration has been seamless."

The cost of the Trump administration's border mission, condemned by critics as a political stunt, is expected to surge to $132 million by the end of this month, CNN reported recently.