WASHINGTON, United States (AFP) — Families crossing the US-Mexico border illegally surged to “crisis” levels in September as policies pressed by the administration of President Donald Trump failed to deter the flow of migrants from Central America, officials said Tuesday.

The jump drove total undocumented immigrant arrests at the southern US frontier in fiscal 2018 to over 520,000, a 25 per cent surge from the previous year.

US Customs and Border Protection (CBP) said it had apprehended 16,658 people who arrived in the country in families last month, some 900 more than August and nearly 12,000 more than September one year ago.

It was the highest month for families on record, reflecting a jump in migrants from Honduras, Guatemala and El Salvador fleeing widespread violence and poverty, many seeking asylum in the United States.

The surge was reported as Trump called a caravan of some 7,000 mostly Honduran migrants planning to traverse Mexico to the southern US border a national emergency and threatened to close the border completely to halt illegal immigration.

His aides blamed the failure of Congress to change laws that make it impossible to repatriate border crossers from Central America and instead force the government to arrest and then release them inside the United States.

CBP said an overall rise in arrests of undocumented border crossers last month helped return illegal immigration numbers to levels seen before Trump became president in January 2017.

For the full fiscal year, which ended September 30, a total of 521,090 people without immigration documents were apprehended or blocked at the Mexico border.

That was up by more than 105,000 from the previous year, Trump’s first year in office when he declared that his tough crackdown on immigration was working.

In fiscal 2016, the last full year before Trump became president, total southern border arrests topped 553,000.

The CBP data showed a continuing trend toward more people crossing in families, despite the Trump administration’s threat to separate parents from their children as a deterrent.

Out of the total last year, families and unaccompanied minors made up about 40 per cent of all those picked up by border officials for trying to sneak into the country.

Speaking under condition of anonymity, a senior administration official called it a border crisis “that is unprecedented in our history.”

The official said laws meant to protect the rights and safety of border crossers are serving as a pull on illegal migrants, allowing them to stay in the United States for years while the US legal system processes their cases.

Simple legal changes — including being able to repatriate Central Americans as is done with Mexican immigrants — will deter many of them from trying, according to the official.

“The cost on society is enormous,” the official said.

“We are apprehending these aliens. If we could return them, there would be no crisis.”