The United States has cut $US700 million ($987 million) in aid to El Salvador, Guatemala and Honduras after US President Donald Trump blasted the Central American countries for over-migration and threatened to shut the US-Mexico border.

Key points: The US State Department says it is following Mr Trump's order to cut Central American aid

The US State Department says it is following Mr Trump's order to cut Central American aid However it appeared to acknowledge that it will need approval from Congress to proceed

However it appeared to acknowledge that it will need approval from Congress to proceed It comes as the US attempts to send back greater numbers of asylum seekers amid a migrant surge

A surge of asylum seekers from the three countries have sought to enter the US across its southern border in recent days, and US officials say border patrol officers have been overwhelmed by the sharp increase in numbers.

Mr Trump has accused the nations of having "set up" migrant caravans and sending them north, threatening that there was a "very good likelihood" he would close the border this week if Mexico did not stop immigrants from reaching the US.

The US is scrambling to relocate border staff to help process asylum seekers. ( AP: Evan Vucci )

And border officials are aiming to more than quadruple the number of asylum seekers sent back over the southern border each day, a Trump administration official said.

Right now, about 60 asylum seekers a day are returned to Mexico but are allowed to return to the US for court dates.

With a backlog of more than 700,000 immigration cases, asylum seekers can wait years for their cases to progress, and officials say some people game the system in order to live in the US.

Officials hope to have as many as 300 people returned per day by the end of the week, focusing particularly on those who come in between ports of entry, said the official, who had knowledge of the plans but was unauthorised to speak publicly.

Mr Trump has repeatedly threatened to close the US-Mexico border during his presidency. ( Supplied: Juan Carlos )

But the process so far has been slow-going, and such a sizeable increase may be difficult to achieve.

A State Department spokesman said in a statement it was carrying out Mr Trump's directive by ending aid programs to the three Central American nations, known as the Northern Triangle.

The department said it would "engage Congress in the process," an apparent acknowledgement that it would need representatives' approval to end funding that a Congressional aide estimated would total about $US700 million.

New Jersey senator Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, called Mr Trump's order a "reckless announcement" and urged Democrats and Republicans alike to reject it.

The Government of El Salvador has said it has tried to stem the flow of migrants.

The Honduran Foreign Ministry called the US policies "contradictory" but stressed that its relationship with the US was "solid, close and positive".

March is on track for 100,000 border apprehensions, according to US officials, which would be the highest monthly number in more than a decade.

Homeland Security secretary Kirstjen Nielsen warned Congress on Thursday that the Government faced a "system-wide meltdown" as it tried to care for more than 1,200 unaccompanied children and 6,600 migrant families in its custody.

Mr Trump has so far been unable to convince Congress to tighten asylum laws or fund his border wall. He has declared a national emergency to justify redirecting money earmarked for the military to pay for the wall.

Reuters/AP