A federal judge has blocked US President Donald Trump from building sections of his border wall with money secured under his declaration of a national emergency.
US District Judge Haywood Gilliam Jr. on Friday immediately halted the administration's efforts to redirect military-designated funds for wall construction.
His order applies to two projects, scheduled to begin as early as Saturday, to replace 82 kilometres of fence in two areas on the Mexican border.
Judge Gilliam issued the ruling after hearing arguments last week in two cases. California and 19 other states brought one lawsuit; the Sierra Club and a coalition of communities along the border brought the other.
His ruling was on the first of several lawsuits against Trump's decision to bypass the normal appropriations process to pay for his long-sought wall.
At stake is billions of dollars that would allow Trump to make progress in a signature campaign promise heading into his campaign for a second term.
Trump declared a national emergency in February after losing a fight with the Democratic-led House over fully paying for the wall that led to a 35-day government shutdown.
As a compromise on border and immigration enforcement, Congress set aside $1.375 billion ($A2 billion) to extend or replace existing barriers in Texas' Rio Grande Valley, the busiest corridor for illegal crossings.
Trump grudgingly accepted the money, but he declared the emergency to siphon money from other government accounts because he wanted to spend $8 billion on wall construction.
The funds include $US3.6 billion from military construction funds, $US2.5 billion from Defense Department counterdrug activities and $US600 million from the Treasury Department's asset forfeiture fund.
The president's adversaries say the emergency declaration was an illegal attempt to ignore Congress, which authorised far less wall spending than Trump wanted.
The administration said Trump was protecting national security as unprecedented numbers of Central American families arrive at the US border seeking asylum.
Australian Associated Press