US President Donald Trump is expected to sign an executive order to build his border wall tomorrow.

People smugglers are telling panicked Central American migrants it is now or never, as Donald Trump prepares to sign an executive order to build his "big, beautiful, powerful wall".

The US president's repeated pledge to build the Mexican border wall appears to be coming to fruition, with Trump likely to order its construction on Thursday (NZT).

Coyotes, or people smugglers, are telling those anxious to flee violence and poverty in Guatemala, Honduras and El Salvador that it is now or never to get into the US.

JONATHAN ERNST Donald Trump has repeatedly promised the barrier, which he says will help keep out unwanted migrants and drugs.

That is causing immigration flows to surge, Alejandra Gordillo, head of the National Council for Migrants at Guatemala's foreign ministry, said.

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Trump has repeatedly promised the barrier, which he says will help keep out unwanted migrants and drugs, and has also pledged to deport undocumented immigrants.

TOMAS BRAVO A US Border patrol agent opens a gate on the fence along the Mexico border to allow vehicles pass in El Paso.

US Border Patrol detained 136,670 migrants along the southwest border in the fourth quarter, up 33 per cent from the same period in 2015.

Big day planned on NATIONAL SECURITY tomorrow. Among many other things, we will build the wall! — Donald J. Trump (@realDonaldTrump) January 25, 2017

"The flow of migrants rose because coyotes are encouraging them," Gordillo said.

TOMAS BRAVO A bag containing the belongings of an undocumented immigrant family after they arrived in the US border city of El Paso on January 17.

"They are telling them that now you can pass," before Trump builds a wall and "they're making it easier for migrants to pay."

Coyotes charge about $6500 to take migrants through Mexico and into the United States, Gordillo said.

For those who don't have the cash, coyotes are extending credits with longer maturities and accepting cars and properties as collateral, she added.