WASHINGTON (AFP) - US southern border detentions plunged for the second straight month in July after a deal with Mexico to block Central American migrants, the Department of Homeland Security announced on Thursday (Aug 8).

US Border Patrol agents detained or blocked 82,049 migrants at the frontier with Mexico last month, down from 104,367 in June and a 13-year peak of 144,266 in May, they said.

The number however was still more than double that of July 2018, and came during the high summer when migrant numbers normally decrease.

The Department of Homeland Security attributed the fall to a deal signed with Mexico in June to stem the flow of migrants traveling northward to the United States from Central America, mainly Guatemala, Honduras and El Salvador.

They also credited cooperation by the three, known as the Northern Triangle countries, in cracking down on migrant smuggling groups.

DHS said the number of detained and blocked Guatemalan migrants fell 41 per cent last month, while Salvadorans were down 21 per cent and Hondurans 16 per cent.

The number of family unit and unaccompanied child apprehensions, which have soared this year, dropped by more than half from May to July, they said.

President Donald Trump issued strong political and trade threats against Mexico and Guatemala in June and July to encourage their cooperation with Washington to halt the migrant flow.

"The situation is improving by every available metric," Acting Secretary of Homeland Security Kevin McAleenan said in a statement.

"But, I want to be very clear, that we remain at and beyond crisis levels in illegal crossings even as our initiatives to address irregular flows and mitigate humanitarian conditions are making an impact," he said.