WASHINGTON • United States President Donald Trump's shock order to send National Guard troops to the frontier with Mexico came after recent data showed that illegal immigration has sharply rebounded following a plunge in his first year in office.

Mr Trump's warning of a crackdown when he entered the White House in January last year drove the number of apprehensions of illegal border crossers - an indicator of total crossing numbers - to four-decade lows.

For example, apprehensions in February last year were 23,555, down from more than 38,000 a year earlier. And they hit a monthly low of 15,766 in April last year, less than one-third of the previous year's number.

At the time, anti-immigration groups celebrated that as the "Trump Effect" and Mr Trump held it up as his great success.

"Jobs are returning, illegal immigration is plummeting, law, order and justice are being restored. We are truly making America great again!" he tweeted that April.

A year later, the data suggests that the Trump Effect lasted barely seven months and that undocumented immigrants are entering the country at a rate similar to 2014-2016, before he ran for president on an anti-immigration platform.

Apprehensions on the south-west border in January and February totalled 72,517, compared to 66,018 a year earlier.

Despite the US boosting border staffing and spending and giving border patrol officers more power, Homeland Security Secretary Kirstjen Nielsen said: "We have recently seen the numbers of illegal border crossings rise from 40-year lows last April back to previous levels."

Ms Nielsen said the rebound comes as human smugglers encourage migrants to take advantage of their knowledge of US laws to avoid quick deportation.

For example, the Homeland Security Department said that one out of 10 people being apprehended by Customs and Border Protection officers asks for asylum based on fear for their lives in dangerous countries like Honduras - compared to one out of 100 in 2013.

Ms Nielsen said that there has been a major jump in recent years in the number of families and unaccompanied children seeking to sneak across the border, and that now half of border crossers are from Central America.

"The traffickers and smugglers know that these individuals cannot, by law, be easily removed back to their country of origin."

AGENCE FRANCE-PRESSE