WASHINGTON (WASHINGTON POST) - Arrests along the Mexico border are projected to fall 25 per cent this month, acting Secretary of Homeland Security Kevin McAleenan said on Friday (June 28), a drop he attributed to a crackdown by Mexican authorities on Central American migrants and the expansion of an experimental programme that requires asylum seekers to wait outside US territory for their immigration court hearings.

US authorities detained more than 144,000 in May, the highest total since 2006.

After President Donald Trump threatened to impose tariffs on Mexican exports, the government of Andres Manuel Lopez Obrador agreed to deploy thousands of troops to intercept migrants and prevent illegal crossings from Mexico into the United States.

"It's become clear that over the past three weeks, since the administration reached a new agreement with Mexico, that we've seen a substantial increase in the number of interdictions on the Mexican southern border, and a sincere effort to the transportation networks coming through Mexico," McAleenan told reporters on Friday in Washington.

The acting secretary praised lawmakers for providing US$4.6 billion (S$6.2 billion) in supplemental funds for border initiatives - a rare bipartisan border agreement reached on Thursday.

He said the funds would immediately be applied to improve conditions in overcrowded processing centres and detention facilities along the southern border.