A US Strikers armored vehicle stands guard at the US-Mexico border in New Mexico on 28 March. Photograph: Hérika Martínez/AFP/Getty Images

Donald Trump has authorized the military to take control of land at the US-Mexico border as part of the president’s broader efforts to crack down on undocumented immigration.

The authorization came late on Friday in a memorandum from Trump to interior secretary Doug Burgum, defense secretary Pete Hegseth, homeland security secretary Kristi Noem and agricultural secretary Brooke Rollins which outlined new policies concerning military involvement at the US’s southern border.

The memorandum, entitled the “Military Mission for Sealing the Southern Border of the United States and Repelling Invasions”, allows the US’s armed forces to “take a more direct role” when it comes to securing the boundary in question.

“Our southern border is under attack from a variety of threats,” the order claimed. “The complexity of the current situation requires that our military take a more direct role in securing our southern border than in the recent past.”

The memorandum added that the Department of Defense should be given jurisdiction to federal lands, including the Roosevelt Reservation, a 60ft-wide strip that stretches over California, Arizona and New Mexico. Doing that would give troops stationed there the legal right to detain immigrants accused of trespassing on what in effect is an elongated base – and unauthorized immigrants would be held in custody until they could be turned over to immigration agents.

Military activities that could be carried out on federal land include “border-barrier construction and emplacement of detection and monitoring equipment”, according to the memorandum.

After 45 days, the defense secretary, Pete Hegseth, will assess the “initial phase” of the order. But at any time, Hegseth could extend the amount of federal land included in the memorandum.

The ordered military takeover excludes native reservations, however, according to the memorandum.

Friday’s order is the latest step from Trump in his administration’s ongoing focus on immigration enforcement, which has involved declaring a national emergency on the southern border.

On Thursday, a US federal judge ruled that the Trump administration was allowed to require people who are in the country but not citizens to register with the federal government, a requirement that advocates say hasn’t been universally implemented since it was enacted as a law in the 1940s.

The ruling comes after the Department of Homeland Security (DHS) announced the new requirement on 25 February, adding that those who failed to report could face fines or possible prison time.

The DHS’s announcement was widely seen as a workaround of the Posse Comitatus Act, a federal law that bars US military troops from participating in most civilian law enforcement actions.

One of the purported justifications for militarizing the US border most commonly cited by Trump and his Republican colleagues is that migrants crossing the border with Mexico without permission carry much of the fentanyl sold in the US. Yet official statistics show 90% of convicted fentanyl peddlers are US citizens.