A US federal judge in San Francisco on Friday halted President Donald Trump's plan to build a wall at the southern border with funds to be diverted from the Department of Defense.

Judge Haywood Gilliam of the US District Court for the Northern District of California issued a temporary injunction, which blocked Trump from planning to redirect money from US military accounts, including pension and pay funds, to cover the cost of a proposed wall at the southern border with Mexico.

The ruling came only one day before the construction of the southern border wall could begin as soon as Saturday, May 25.

The judge ruled against the transfer of about 1 billion US dollars in military pay and pension funds to pay for the wall.

Gilliam made the ruling after hearing arguments last week involving two lawsuits - one brought by California and 19 other states and the other filed by the American Civil Liberties Union (ACLU) on behalf of the Southern Border Communities Coalition, a civil rights alliance, and the Sierra Club environmental NGO.

Both cases challenged the legality of Trump's plan for funding the border barrier construction, which may exceed "the Executive Branch's lawful authority under the Constitution and a number of statutes duly enacted by Congress," Gilliam said in the ruling.

Trump declared a national emergency in February after failing to obtain full funding from the Democratic-dominated House, vowing to divert money from other government agencies.

Gilliam's ruling was welcomed by the ACLU.

"This is a win for our system of checks and balances, the rule of law, and border communities. The court blocked all wall projects currently slated for immediate construction," the ACLU tweeted late Friday.

"If the Trump administration begins illegally diverting additional funds, we'll be back in court," it added.