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

US troops deployed to the US-Mexico border will remain there until at least the end of September 2019, the Pentagon revealed in an emailed statement Jan. 14, 2019.

Acting Secretary of Defense Patrick Shanahan, who took over for former Secretary of Defense James Mattis at the beginning of 2019 has approved Department of Defense assistance to the Department of Homeland Security (DHS) through Sept. 30, 2019.


The decision was made in response to a DHS request submitted in late December 2018.

The initial deployment, which began in October 2018 as "Operation Faithful Patriot" (since renamed "border support"), was expected to end on Dec. 15, 2018. The mission had previously been extended until the end of January 2019.

U.S. Marines with the 7th Engineer Support Battalion, Special Purpose Marine Air-Ground Task Force 7, walk along the California-Mexico border at the Andrade Point of Entry in Winterhaven, California, Nov.30, 2018.

(U.S. Army photo by Spc. Ethan Valetski)

Thousands of active-duty troops, nearly six thousand at the operation's 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.

The Department of Defense is transitioning the support provided from securing ports of entry to mobile surveillance and detection activities, according to the Pentagon's emailed statement. Troops will offer aviation support, among other services.

Shanahan has 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.

U.S. Marines with 7th Engineer Support Battalion, Special Purpose Marine Air-Ground Task Force 7, secure concertina and barbed wire near the California-Mexico border at the Andrade Port of Entry in California, Nov. 29, 2018.

(U.S. Marine Corps photo by Sgt. Asia J. Sorenson)

The extension of the border mission was expected after 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 early January 2019.

"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 rise to $132 million by the end of this month, CNN reported recently.

This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.