WASHINGTON (Reuters) - The number of illegal immigrants crossing into the United States from Mexico declined by 40 per cent from January to February, Homeland Security Secretary John Kelly said on Wednesday (March 8).

The downturn came after President Donald Trump took office on Jan 20 vowing to deport many of the estimated 11 million illegal immigrants in the United States.

The flow of illegal border crossings as measured by apprehensions and the prevention of inadmissible persons at the southern border dropped to 18,762 persons in February from 31,578 in January, Kelly said in a statement.

He said the US Customs and Border Protection agency, which compiled the data, historically sees a 10 per cent to 20 per cent increase in apprehensions of illegal immigrants from January to February.

On Jan 25, Trump ordered the construction of a wall along the roughly 2,000-mile (3,200-km) US-Mexico border, moved to strip federal funding from "sanctuary" states and cities that harbour illegal immigrants, and expanded the force of US immigration agents.

"Since the administration's implementation of Executive Orders to enforce immigration laws, apprehensions and inadmissible activity is trending toward the lowest monthly total in at least the last five years," Kelly said.