US President Donald Trump's plans hit a snag on Friday with the filing of the first lawsuit against the national emergency he has declared to secure funding for a southern border wall.

Progressive non-profit organization Public Citizen said in a statement it had filed the action in a federal district court in Washington on behalf of three Texas landowners and Frontera Audubon Society, a nature park in the Rio Grande Valley.

‘We just sued Trump over his fake national emergency,’ Public Citizen said.

‘If Trump gets away with this, there's no telling what the next concocted 'emergency' will be, who will be targeted and what emergency powers will be claimed. We refuse to tolerate this slide to authoritarianism.’  They also argued their property would either be swallowed up by future wall construction or, in the case of the park, that rare birds would disappear from the area.

Critics have said that Trump's difficulties with Congress do not meet the criteria of the 1970s National Emergencies Act.

They say Trump's admission in his Friday announcement that ‘I don't need to do this’ showed he was not justified in redirecting 3.6 billion dollars from military construction to the border wall.

‘I can do a wall over a longer period of time. I don't need to do this, but I'd rather do it much faster,’ Trump said earlier Friday outside the White House.

Friday's move enables the president to access money to build hundreds of kilometres of physical barriers on the southern frontier.

Trump will use 8 billion dollars for the wall, of which 1.375 billion was appropriated by Congress for barriers, far below the president's demand of 5.7 billion to build some 377 kilometres of wall.

The rest will come from executive action and the national emergency declaration, split about evenly, by taking money from the Pentagon's construction budget and drug forfeitures.

Trump campaigned on the wall and pledged Mexico would pay for it, something that has not materialized. He was also once a fierce critic of former president Barack Obama when he evaded Congress by taking executive action.