WASHINGTON, D.C. — President Trump, unable to get Congress or Mexico to fully fund his border wall, will post National Guard troops along the Mexican frontier, officials said on Wednesday, in a move that was likely to escalate tensions with a key U.S. ally.

The Trump administration was working with the governors of the four southwestern U.S. states along the border to deploy the Guard, said Homeland Security Secretary Kirstjen Nielsen, adding that the troops would not be involved in law enforcement.

In a supporting role, possibly for aerial reconnaissance, the Guard will help U.S. Customs and Border Protection personnel with stopping illegal immigrants from entering the country, Nielsen said at a White House briefing with reporters.

In a memorandum laying out the new initiative, Trump directed Defense Secretary James Mattis to request the use of National Guard personnel to help the Department of Homeland Security in securing the southern border.

He ordered Mattis, Nielsen and Attorney General Jeff Sessions to submit a report within 30 days detailing an action plan and recommendations for any other executive authorities to be invoked to protect the border.

While the Trump administration speaks of an immigration “crisis” on the border, U.S. Border Patrol statistics show the fewest apprehensions of illegal immigrants on the border in 46 years. In the fiscal year that ended Sept. 30, 2017, there were 303,916 such arrests, the lowest level since fiscal 1971.