President Donald Trump has threatened to push the US federal government into shut down if Democrats refuse to back his demand for a wall at the Mexican border and other major changes to immigration laws.

“I would be willing to ‘shut down’ government if the Democrats do not give us the votes for Border Security, which includes the Wall!,” Mr Trump tweeted Sunday night (AEST).

The President railed against the US green card lottery system, as well as what he called the policy of “Catch & Release” applied to migrants caught crossing the US-Mexico border.

I would be willing to “shut down” government if the Democrats do not give us the votes for Border Security, which includes the Wall! Must get rid of Lottery, Catch & Release etc. and finally go to system of Immigration based on MERIT! We need great people coming into our Country!

— Donald J. Trump (@realDonaldTrump) July 29, 2018

The Republican president has used the threat of a government shutdown due to lack of funding supply several times since taking office in 2017 in a bid to get his priorities in congressional spending bills, especially funding for a wall along the southern US border.

A disruption in federal government operations in the months before November congressional elections could backfire on Mr Trump if voters blame Republicans, who control Congress, for the interruption in services.

The President’s latest threat came several days after the government said more than 1800 children separated at the US-Mexico border have been reunited with parents and sponsors after a federal judge ordered the reunions. Hundreds of children still remain separated.

“Please understand, there are consequences when people cross our Border illegally, whether they have children or not – and many are just using children for their own sinister purposes. Congress must act on fixing the DUMBEST & WORST immigration laws anywhere in the world!” he added.

Mr Trump wants Congress to pass legislation that addresses immigration issues, including the border wall, changing the way visas are allotted and other immigration restrictions.

Although Republicans control Congress, disagreements between moderates and conservatives in the party have impeded a speedy legislative fix.

Stand-offs over spending levels and immigration led to a three-day government shutdown, mostly over a weekend, in January and an hours-long shutdown in February.

The Republican president has made tougher immigration laws a centrepiece of his administration, from the first ill-fated travel ban on people from predominantly Muslim nations to the current battle raging over the separation of illegal immigrant children from their parents at the US-Mexico border.

A federal judge on Friday urged the government to focus on finding deported immigrant parents so it could reunite them with their children who remain in the United States.

Mr Trump has requested $US25 billion ($33.8 billion) to build the border wall and $US1.6 billion has already appropriated for the project.

-with AAP