Washington (AFP) - Illegal immigration across the US-Mexico border fell for the fifth straight month in April as the Trump administration's crackdown takes root, the Department of Homeland Security said Tuesday.

Apprehensions of undocumented immigrants, an indicator for the overall flow, fell to 11,129 in April, the lowest in decades, down two-thirds from the same month a year ago.

In the first three full months since President Donald Trump's inauguration on January 20, just over 42,000 were detained, compared to nearly 133,000 a year ago.

DHS spokesman David Lapan credited tougher surveillance and enforcement by the US Customs and Border Protection agency, but said they anticipate a possible uptick in coming months after the winter.

The data showed a sharp fall in the number of unaccompanied children attempting to enter the country illegally, 998 in April compared to more than 7,000 in both November and December.

Despite the fall in numbers Lapan said DHS is still seeking to add as many as 5,000 more immigration enforcement staff.

"We still have shortages" in border staff numbers, he said.