Trump, stymied on wall, to send troops to U.S.-Mexico border

(Adds Trump signing memorandum on planned deployment, Mexican reaction)

WASHINGTON, April 4 (Reuters) - President Donald Trump, unable to get the U.S. 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.

The administration's move drew criticism from Democrats. Senator Michael Bennet of Colorado said Trump has failed to engage with lawmakers on bipartisan immigration reform that would satisfy both parties' agendas on the volatile issue.

"Unfortunately, the president failed to lead, and rather than find real solutions on immigration, he continues to stoke fear," Bennet said in a statement.

The Mexican government has told the United States that "if the announced deployment of the National Guard turned into a militarization of the border, that would gravely damage the bilateral relationship," Mexico's Foreign Ministry said.

It said Nielsen discussed the planned National Guard deployment with Mexican Foreign Minister Luis Videgaray on Wednesday and told him the troops will not carry arms.

In keeping with a theme he often invoked as a candidate in 2016 and has continually returned to since taking office, Republican Trump has sharpened his anti-immigrant rhetoric, warning that illegal immigrants threaten U.S. safety and jobs.

His plan to deploy troops comes after his failure so far to persuade either the Mexican government or the U.S. Congress to fully fund a wall he wants to build along the border.

At the same time, the Republican-controlled Congress has failed to meaningfully overhaul U.S. immigration law, despite demands from Trump for a deal. With campaigning by lawmakers for November's midterm congressional elections getting under way, little legislative action was expected in months ahead.