WASHINGTON - President Donald Trump, unable to get the US 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 US ally.The Trump administration was working with the governors of four southwestern US 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 US Customs and Border Protection personnel with stopping illegal immigrants from entering the country, Nielsen said at a White House briefing with reporters.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.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 US safety and jobs.His plan to deploy troops comes after his failure so far to persuade either the Mexican government or the US 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 US immigration law, despite demands from Trump for a deal. With campaigning by lawmakers for November's midterm congressional elections getting underway, little legislative action was expected in months ahead."The president has directed that the Department of Defense and the Department of Homeland Security work together with our governors to deploy the National Guard to our southwest border to assist the Border Patrol," Nielsen said. "The president will be signing a proclamation to that effect today."The National Guard is a reserve wing of the US armed forces that is partially under the authority of governors.Trump's plans were hailed as welcome and needed by the Republican governors of Arizona and Texas.The California National Guard will promptly review Trump's request "to determine how best we can assist our federal partners," said a spokesman for the state's unit in a statement. He added he was speaking for Governor Jerry Brown, a Democrat.