The United States will expand the scope of a controversial programme that returns non-Mexican migrants and asylum seekers crossing Mexico’s border to that country to await their US court hearings, the Department of Homeland Security said on Thursday.

Washington already has sent more than 56,000 migrants to Mexico under the programme, known as the Migrant Protection Protocols (MPP). The majority have been Central Americans applying for US asylum.

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The effort - which had been implemented over the past year at ports of entry in Texas and California — will be applied at a port of entry south of Tucson, Arizona. Previously, migrants encountered in that area had been sent to El Paso, Texas, to process their return to Mexico.

US President Donald Trump has made a crack down on immigration a major focus of his first term in office and has continued to press the issue in the run-up to the 2020 presidential election.

Acting DHS Secretary Chad Wolf said in a written statement on Thursday that the MPP programme has been “an extremely effective tool”.

Immigration advocates counter that the initiative exposes migrants to violence in Mexico and restricts their ability to seek protections in the United States. The US consulate in Mexico’s border city of Nuevo Laredo issued a security alert on Wednesday, warning against gun battles and urging government employees to take precautions.

The consulate continues to monitor the security situation in Nuevo Laredo. US government personnel are subject to enhanced restrictions on their movements, earlier evening curfew than normal until further notice. Assistance: +528677140512, +525550802000, nuevolaredo-acs@state.gov — USCG Nvo Laredo (@USAConNVL) January 2, 2020

Mexico’s asylum agency, known as COMAR, said on Wednesday that it had received 66,915 applications for asylum in 2019, up nearly 126 percent from the previous year.

The agency has struggled to process the crush of applications with limited resources. COMAR received 20 million pesos ($1 million) in funding from the Mexican government in 2019, the lowest sum in seven years.

In December, US border authorities expanded a separate programme to process asylum cases rapidly to the Rio Grande Valley in Texas, according to CBS News. The programme, known as Prompt Asylum Claim Review (PACR), had been launched months earlier as a pilot in the El Paso area.

US authorities have not specified the criteria to place migrants in a growing array of border programmes, which include MPP, PACR and another initiative that sends asylum seekers from Honduras and El Salvador to request protection in Guatemala.

Rodney Scott, acting deputy chief of the US Border Patrol, told reporters in December that the factors “change on a daily basis” but are determined in coordination with the receiving countries.