US President Donald Trump's national emergency declaration to divert more than $6 billion to build a wall on the border with Mexico was set to face its first test in court on Friday, when states and advocacy groups were expected to ask a federal judge to block the funding.

At the center of Friday's hearing is the question of the president's authority to construct a wall using funds that Congress has declined to approve for the amount he requested.

In February, Congress approved $1.375 billion for construction of "primary pedestrian fencing" along the border in southeast Texas, well short of Trump's demand for $5.7 billion to build border walls in Arizona and New Mexico as well as Texas.

To obtain the additional money, Trump declared a national emergency and diverted $601 million from a Treasury forfeiture fund, $3.6 billion from military construction and $2.5 billion earmarked for Department of Defense counterdrug programs.

"Congress's refusal to fund President Trump's wall isn't an emergency, it's democracy," said a statement from Dror Ladin, an attorney for the American Civil Liberties Union, which represents plaintiffs in the case.

The Trump administration argues the plaintiffs have not shown any injury caused by the funding decisions and that existing law gives it the leeway to redirect the money for such purposes as "an unforeseen military requirement" or a "law enforcement activity."